[libc] [llvm] [libc] Move libc_errno.h to libc/src/__support and make LIBC_ERRNO_MODE_SYSTEM to be header-only. (PR #143187)

[Alexey Samsonov via llvm-commits]

https://github.com/vonosmas approved this pull request.

LGTM with a tiny nit.

https://github.com/llvm/llvm-project/pull/143187