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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

There's a latent inclusion of `src/__support/macros/config.h` in libc/src/__support/macros/optimization.h that looks like it can be removed.  We can save that for another PR; I've filed https://github.com/llvm/llvm-project/issues/86579 for now.

It's ok to add commits on top rather than amend+force push; this helps reviewers see better what changed between revisions.

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