gh-122931: Allow stable abi3 API extensions to include a multiarch tuple in the filen...

gh-122931: Allow stable abi3 API extensions to include a multiarch tuple in the filename (GH-152461)

This permits stable ABI extensions for multiple architectures to be
co-installed into the same directory, without clashing with each other,
the same way (non-stable ABI) regular extensions can.

It is listed before the current platform-less suffixes since it's more
specific.

The platform is stored in a new pyconfig.h define & sysconfig
variable, SOABI_PLATFORM.
On some known architectures (FreeBSD, Windows), this
will be undefined/zero; these won't have the new tag (yet).

Add SOABI_PLATFORM & ALT_SOABI the info to
`make pythoninfo` as well.

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>