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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

I'm fat+lazy.

JK, but It's complicated.  I'm not sure that llvm even endorses/requires bazel to build, but if we land changes to llvm-libc that break the build with bazel, my coworkers yell at me.

Getting bazel runs in presubmit is a good idea though.

Filed: https://github.com/llvm/llvm-project/issues/86601

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