[llvm] [libc][bazel] Convert "errno" to libc_support_library. (PR #130368)

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lntue wrote:

Currently, bazel build is essentially in overlay mode.  So I think we should define `-DLIBC_ERRNO_MODE=LIBC_ERRNO_MODE_SYSTEM` for the errno target.  Can you try to see if it's fine for our tests?

https://github.com/llvm/llvm-project/pull/130368