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

[Marc Auberer via llvm-commits]

marcauberer 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.

Should be fixed now. Why is this not covered by the test runs on GH?

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