Edk2ToolsBuild.py: set arch to host arch if not specified on linux (#381

)

# Preface

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## Description

The -a <ARCH> command line options should be optional, defaulting to
IA32 on windows or the host arch on linux, however this scenario fails
on linux systems as the arch is not set to the host's architecture. This
PR adds this logic.

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- [ ] Impacts functionality?
- **Functionality** - Does the change ultimately impact how firmware
functions?
- Examples: Add a new library, publish a new PPI, update an algorithm,
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- [ ] Impacts security?
- **Security** - Does the change have a direct security impact on an
application,
    flow, or firmware?
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    validation improvement, ...
- [ ] Breaking change?
- **Breaking change** - Will anyone consuming this change experience a
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    in build or boot behavior?
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    a function in a new library class in a pre-existing module, ...
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## How This Was Tested

<_Please describe the test(s) that were run to verify the changes._>

## Integration Instructions

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required._>

BaseTools/Edk2ToolsBuild.py

@@ -203,6 +203,7 @@ def Go(self):
             # MU_CHANGE STARTs: Specify target architecture
             # Note: This HOST_ARCH is in respect to the BUILT base tools, not the host arch where
             # this script is BUILDING the base tools.
+            HostInfo = GetHostInfo()
             prefix = None
             TargetInfoArch = None
             if self.target_arch is not None:
@@ -228,11 +229,12 @@ def Go(self):
 
                 else:
                     TargetInfoArch = "x86"
-
+            else:
+                self.target_arch = HostInfo.arch
+                TargetInfoArch = HostInfo.arch
             # Otherwise, the built binary arch will be consistent with the host system
 
             # Added logic to support cross compilation scenarios
-            HostInfo = GetHostInfo()
             if TargetInfoArch != HostInfo.arch:
                 # this is defaulting to the version that comes with Ubuntu 20.04
                 ver = shell_environment.GetBuildVars().GetValue("LIBUUID_VERSION", "2.34")