Issue 42688: ctypes memory error on Apple Silicon with external libffi

Building python 3.9.1 on Apple Silicon compiled against a external (non-os-provided) libffi makes the following code return a MemoryError:

Test:

import ctypes

@ctypes.CFUNCTYPE(None, ctypes.c_int, ctypes.c_char_p)
def error_handler(fif, message):
    pass

I have tracked this down to the following code in malloc_closure.c:

#if USING_APPLE_OS_LIBFFI && HAVE_FFI_CLOSURE_ALLOC
    if (__builtin_available(macos 10.15, ios 13, watchos 6, tvos 13, *)) {
        ffi_closure_free(p);
        return;
    }
#endif

and

#if USING_APPLE_OS_LIBFFI && HAVE_FFI_CLOSURE_ALLOC
    if (__builtin_available(macos 10.15, ios 13, watchos 6, tvos 13, *)) {
        return ffi_closure_alloc(size, codeloc);
    }
#endif

In fact, while the __builtin_available() call should be guarded by USING_APPLE_OS_LIBFFI, the call to ffi_closure_alloc() should only be guarded by HAVE_FFI_CLOSURE_ALLOC, as this is set as the result of an independent check in setup.py and should be used with external libffi when supported.

The following code does work instead:

#if HAVE_FFI_CLOSURE_ALLOC
#if USING_APPLE_OS_LIBFFI
    if (__builtin_available(macos 10.15, ios 13, watchos 6, tvos 13, *)) {
#endif
        ffi_closure_free(p);
        return;
#if USING_APPLE_OS_LIBFFI
    }
#endif
#endif


#if HAVE_FFI_CLOSURE_ALLOC
#if USING_APPLE_OS_LIBFFI
    if (__builtin_available(macos 10.15, ios 13, watchos 6, tvos 13, *)) {
#endif
        return ffi_closure_alloc(size, codeloc);
        return;
#if USING_APPLE_OS_LIBFFI
    }
#endif
#endif

Thanks for your quick feedback!  I signed the CLA after submitting the PR, but I think it takes a bit of time to percolate through the system.

As for the "why", until 3.9.1 conda-forge had been successfully using an external ffi (with 3.9.0 + osx-arm64 patches) and then suddenly it broke.  For the time being conda-forge is using the system ffi, which does have other advantages as well, having all the latest patches.  However, I was curious as to _why_ it broke, and that led me to discover this bug, and it seemed straightforward to fix.  When/if conda-forge will switch back to external ffi is TBD, but if that decision is made this issue (at least) will be taken care of.