[clang] [Driver] Teach Barmetal toolchain about GCC installation (PR #121829)

[Peter Smith via cfe-commits]

https://github.com/smithp35 commented:

I've done another review pass. Mostly some small stylistic comments.

I'm finding it difficult to follow the code with the various different code-paths with similar names. I've made some suggestions on renaming. We may also be able to add some more comments to make it clearer.

https://github.com/llvm/llvm-project/pull/121829