gh-99337: Fix compile errors with gcc 12 on macOS (#99470)

Fix a number of compile errors with GCC-12 on macOS:

1. In pylifecycle.c the compile rejects _Pragma within a declaration
2. posixmodule.c was missing a number of ..._RUNTIME macros for non-clang on macOS
3. _ctypes assumed that __builtin_available is always present on macOS