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

[Keith Smiley via llvm-commits]

keith wrote:

good to know thanks! I'm not sure how much more I will try to do, originally I just set out to do a `bazel build ...` and started dealing with the failures, but I'm not sure if we will have a dep on this that will require more fixes, we'll see!

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