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

[Alexey Samsonov via llvm-commits]

vonosmas 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?

I believe this is orthogonal to the PR, since the PR only changes the bazel plumbing, but doesn't modify how the library is used in any meaningful way?

In any case, I looked at libc/src/errno/libc_errno.cpp , and it seems that:
* for release build, the default would already be `-DLIBC_ERRNO_MODE=LIBC_ERRNO_MODE_SYSTEM`, since `LIBC_FULL_BUILD` is not defined, but `LIBC_COPT_PUBLIC_PACKAGING` is.
* for unit tests, the default would be `-DLIBC_ERRNO_MODE=LIBC_ERRNO_MODE_THREAD_LOCAL`. If I explicitly set it to `LIBC_ERRNO_MODE_SYSTEM`, the tests still pass. Let me know if you'd like me to do it, but I'd prefer to land it in a separate CL.

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