[libc] [llvm] [libc] Remove obsolete LIBC_HAS_BUILTIN macro (PR #86554)

[Nick Desaulniers via llvm-commits]

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

@marcauberer thanks for the patch? Are you able to land this or do you need one of us to?

https://github.com/llvm/llvm-project/pull/86554