Select the PyObject model used by cpyext's rawrefcount: store ``ob_pypy_link``
in a hidden word before ``ob_refcnt`` instead of as a visible header field, and
make the ``REFCNT_FROM_PYPY`` tag permanent.  This makes the visible PyObject
header identical to CPython's so that abi3 wheels can be loaded.  Only a
cpyext whose allocator reserves the prefix word may turn this on.  See
``pypy/doc/discussion/rawrefcount.rst``.
