The macOS 13 SDK includes support for the ``mkfifoat`` and ``mknodat`` system calls.
Using the ``dir_fd`` option with either :func:`os.mkfifo` or :func:`os.mknod` could result in a
segfault if cpython is built with the macOS 13 SDK but run on an earlier
version of macOS. Prevent this by adding runtime support for detection of
these system calls ("weaklinking") as is done for other newer syscalls on
macOS.
