bpo-41617: Add _Py__has_builtin() macro (GH-23260)

Fix building pycore_bitutils.h internal header on old clang version
without __builtin_bswap16() (ex: Xcode 4.6.3 on Mac OS X 10.7).

Add a new private _Py__has_builtin() macro to check for availability
of a preprocessor builtin function.

Co-Authored-By: Joshua Root <jmr@macports.org>

Co-authored-by: Joshua Root <jmr@macports.org>