[libc] Remove obsolete LIBC_HAS_BUILTIN macro (#86554)

Fixes #86546 and removes the macro `LIBC_HAS_BUILTIN`. This was
necessary to support older compilers that did not support
`__has_builtin`. All of the compilers we support already have this
builtin.
See: https://libc.llvm.org/compiler_support.html
All uses now use `__has_builtin` directly

cc @nickdesaulniers

GitOrigin-RevId: 77118536b52bf5256eed85f61451d0beb6cf5dc3
25 files changed