gccWithoutTargetLibc: if glibc, enable threads and pass headers

[ghost]

⚠️ see update below ⚠️

Description of changes

If ${targetPackages.glibc}/include/stdio.h does not exist during the configurePhase of pkgsCross.*.gcc.libgcc, the stack unwinder will be silently disabled.

The resulting libgcc_s.so will deliberately crash any process which attempts to use pthread_cleanup_push() or similar calls.

This PR ensures that ${targetPackages.glibc}/include/stdio.h does exist during the configurePhase of pkgsCross.*.gcc.libgcc, so we get an unwinder-capable libgcc_s.so.

This also adds a new package glibcHeaders, which allows to build the glibc headers without first having built a targetPlatform compiler. This is needed in order to break the dependency cycle between glibc and libgcc. Additional details are in the commit message for that particular commit

Closes:

• mbuffer: cross-compiled libgcc_s.so.1 must be built with glibc headers for pthread_cleanup_push() to work #213453

Includes

• gcc: tighten condition for inhibit_libc=true #241206
• gccCrossStageStatic: enable dynamic libraries, rename it #238154

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[ghost]

Update: so, this does fix #213453, and it does work on x86_64 and aarch64. Unfortunately it pretty much doesn't work on any other architecture.

Apparently being able to ./configure without a targetPlatform compiler is a fairly new feature for glibc and it has only been implemented on the popular platforms. So I don't think this PR will ever be undraftified (and, because of that, I am unlikely to hunt down the OfBorg eval failure although I can rebuild my whole userspace without hitting it).

After much digging around in the guts of glibc I think I've found a better solution: right now we use the libgcc from the first-stage gcc (aka gccWithoutTargetLibc). That libgcc is missing the unwinder. All we need to do is use the libgcc from the final gcc instead.

There are two ways to do this:

1. The easy way: add an extra recompile of glibc for pkgsCross.*.stdenv. This is simple enough that it might be backport-eligible.

2. The better way: use a technique similar to patchelf --add-rpath to update the pointer in glibc. Of course the pointer-updated glibc would be a new derivation with a new outpath, but unlike in the "easy way" it wouldn't require any compiling. Most importantly, it is a change which could be applied to both pkgsCross.*.stdenv and also to native-stdenv. This will take time, testing, and have to go through staging. We can't use patchelf since the "pointer" isn't an ELF DT_NEEDED, but it is a simple string in the static-data section of ld.so.

So to summarize:

1. This PR is 99%-likely to be abandoned.
2. I will write a PR for strategy 1 above in the next week or two, and if merged propose it for backporting.
3. I will write another PR which reverts strategy 1 and implements strategy 2 sometime in the next few months. This PR will not be proposed for backporting to 23.05.