[build] Apple Silicon M1 build support

Fixes: dotnet#5518

Context: https://github.com/xamarin/xamarin-android/projects/16

[Apple announced new Mac hardware][0], which use an [M1 CPU][1], which
is an arm64-compatible ABI, *not* intel based.  Legacy software built
for intel CPUs will still run via the [Rosetta][3] translation
environment.

Because of Rosetta, existing Xamarin.Android binaries will work as-is
on Mac hardware with M1 CPUs.

However, what if you want to *build* the xamarin-android repo on an
M1 CPU?

Previously, that would fail:

	% make prepare PREPARE_AUTOPROVISION=true
	…
	  Error: Could not find Homebrew on this system, please install it from https://brew.sh/

Unfortunately, [Homebrew][4] won't install by default:

	% /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"

	Password:
	Homebrew is not (yet) supported on ARM processors!
	Rerun the Homebrew installer under Rosetta 2.
	If you really know what you are doing and are prepared for a very broken
	experience you can use another installation option for installing on ARM:
	  https://docs.brew.sh/Installation

However, there *is* a way to install Homebrew!  You "just" need to
"lie" about the processor being used!

	% arch -arch x86_64 /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
	# works!

This works by using the [**arch**(1)][5] command, which allows
explicitly specifying (overriding) the CPU environment that the child
process runs within.  Within `arch -arch x86_64 command`, `command`
believes it's running on an x86_64 CPU.

Update the Unix `dependencies.md` documentation to document how to
install Homebrew on an M1 CPU.

Update `xaprepare` so that whenever a `brew` command is executed on
an M1 CPU, the command is prefixed with `arch -arch x86_64`.

`xaprepare` uses the [`sysctl.proc_translated` system control][6]
to determine if it's currently running within a Rosetta environment,
so that `arch -arch x86_64` is not used unless required.

[0]: https://www.apple.com/newsroom/2020/11/introducing-the-next-generation-of-mac/
[1]: https://www.apple.com/newsroom/2020/11/apple-unleashes-m1/
[2]: https://www.apple.com/newsroom/2020/11/apple-unleashes-m1/
[3]: https://developer.apple.com/documentation/apple_silicon/about_the_rosetta_translation_environment
[4]: https://brew.sh
[5]: https://www.unix.com/man-page/osx/1/arch/
[6]: https://developer.apple.com/documentation/apple_silicon/about_the_rosetta_translation_environment#3616845

Documentation/building/unix/dependencies.md

@@ -2,6 +2,7 @@
 
 Building Xamarin.Android requires:
 
+  * [Homebrew](#homebrew)
   * [Latest Mono](#mono-sdk)
   * [The Java Development Kit (JDK)](#jdk)
   * [Autotools (`autoconf`, `automake`, etc.)](#autotools)
@@ -28,6 +29,18 @@ to provide install instructions to obtain the missing dependency, e.g.:
 
     error : Could not find required program '7za'. Please run: brew install 'p7zip'.
 
+<a name="homebrew" />
+
+## Homebrew
+
+[Homebrew](https://brew.sh) must be installed and available via `$PATH` in
+order to provision xamarin-android.
+
+When building on Apple Silicon (arm64) machines, use the **arch**(1) command to
+allow Homebrew to be installed:
+
+	% arch -arch x86_64 /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
+
 
 <a name="mono-sdk" />
 

build-tools/xaprepare/xaprepare/Application/GeneratedMakeRulesFile.cs

@@ -39,6 +39,7 @@ void Generate (Context context, StreamWriter sw)
 
 			WriteVariable ("export OS_NAME", context.OS.Type);
 			WriteVariable ("export OS_ARCH", context.OS.Architecture);
+			WriteVariable ("export OS_ARCH_TRANSLATED", context.OS.ProcessIsTranslated ? "true" : "false");
 			WriteVariable ("PRODUCT_VERSION", context.ProductVersion);
 			WriteVariable ("MONO_SOURCE_FULL_PATH", Configurables.Paths.MonoSourceFullPath);
 

build-tools/xaprepare/xaprepare/OperatingSystems/MacOS.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Runtime.InteropServices;
 
 namespace Xamarin.Android.Prepare
 {
@@ -26,6 +27,10 @@ brew doctor
 		public Version HomebrewVersion { get; private set; }
 		public bool HomebrewErrors     { get; set; }
 
+		bool processIsTranslated;
+
+		public override bool ProcessIsTranslated => processIsTranslated;
+
 		public MacOS (Context context)
 			: base (context)
 		{
@@ -52,9 +57,21 @@ public MacOS (Context context)
 				Version = new Version (0, 0, 0);
 			}
 
+			processIsTranslated = GetProcessIsTranslated ();
+
 			Dependencies = new List<Program> ();
 		}
 
+		unsafe bool GetProcessIsTranslated ()
+		{
+			int ret = 0;
+			ulong size = sizeof (int);
+			if (NativeMethods.sysctlbyname ("sysctl.proc_translated", &ret, &size, null, 0) == -1) {
+				return false;
+			}
+			return ret != 0;
+		}
+
 		protected override void PopulateEnvironmentVariables ()
 		{
 			base.PopulateEnvironmentVariables ();
@@ -104,4 +121,10 @@ public override void ShowFinalNotices ()
 			Log.WarningLine (HomebrewErrorsAdvice, ConsoleColor.White, showSeverity: false);
 		}
 	};
+
+	partial class NativeMethods {
+		[DllImport ("c")]
+		internal static unsafe extern int sysctlbyname (
+			string name, void* oldp, ulong *oldlenp, void* newp, ulong newlen);
+	}
 }

build-tools/xaprepare/xaprepare/OperatingSystems/OS.cs

@@ -133,6 +133,8 @@ abstract class OS : AppObject
 		/// </summary>
 		public string HomebrewPrefix { get; set; } = String.Empty;
 
+		public virtual bool ProcessIsTranslated => false;
+
 		/// <summary>
 		///   File extension used for ZIP archives
 		/// </summary>

build-tools/xaprepare/xaprepare/ToolRunners/BrewRunner.MacOS.cs

@@ -11,6 +11,7 @@ partial class BrewRunner : ToolRunner
 		static readonly char[] lineSplit = new [] { '\n' };
 
 		bool? needSudo;
+		bool? needArch;
 
 		protected override string DefaultToolExecutableName => GetDefaultExecutableName ();
 		protected override string ToolName                  => "Homebrew";
@@ -34,9 +35,15 @@ string GetDefaultExecutableName ()
 					throw new InvalidOperationException ($"BrewRunner does not suppport {Context.Instance.OS.Name}");
 
 				needSudo = os.HomebrewVersion != null && os.HomebrewVersion < sudoVersion;
+				needArch = os.ProcessIsTranslated;
 			}
 
-			return needSudo.Value ? "sudo" : BrewPath;
+			if (needSudo ?? false)
+				return "sudo";
+			if (needArch ?? false)
+				return "arch";
+
+			return BrewPath;
 		}
 
 		public async Task<bool> Tap (string tapName, bool echoOutput = true, bool echoError = true)
@@ -159,7 +166,17 @@ ProcessRunner GetBrewRunner (bool echoOutput, bool echoError, List<string> argum
 		{
 			ProcessRunner runner = CreateProcessRunner ();
 
-			if (needSudo.HasValue && needSudo.Value) {
+			if ((needSudo ?? false) && (needArch ?? false)) {
+				// So we run `sudo arch -arch x86_64 brew …`
+				runner.AddArgument ("arch");
+			}
+
+			if (needArch ?? false) {
+				runner.AddArgument ("-arch");
+				runner.AddArgument ("x86_64");
+			}
+
+			if ((needSudo ?? false) || (needArch ?? false)) {
 				runner.AddArgument (BrewPath);
 			}