[llvm] [bazel] Improve libc build on macOS (PR #86174)

[Michael Jones via llvm-commits]

https://github.com/michaelrj-google approved this pull request.

This patch LGTM, I will warn you that there hasn't been much work on the MacOS build (bazel or cmake) recently so there may be some broken pieces. Feel free to send issues my way, but also we could absolutely use your expertise in bringing it online.

https://github.com/llvm/llvm-project/pull/86174