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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

> I'll wait for presubmit results and @gchatelet 's feedback when he's comes back online, but LGTM.

There's some bazel breakage from this change. Let me figure out what's going on.

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