[libcxx] [libcxxabi] [libunwind] [runtimes] Reintroduce a way to select the compiler used for the test suite (PR #93542)

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https://github.com/EricWF approved this pull request.

Thanks for doing this.

I know from trying this yesterday than some of the configuration options surrounding `clang-tidy` don't tolerate this (in particular when switching from clang->gcc), but I'll send a separate change to address that.

https://github.com/llvm/llvm-project/pull/93542