Issues #22980, 23969: For OS X, use PEP 3149-style file names for extension

Issues #22980, 23969: For OS X, use PEP 3149-style file names for extension
module binaries, with a platform triple of just "darwin", resulting in
file names like:

    _ssl.cpython-35m-darwin.so

rather than just _ssl.so as previously.

Instead of attempting to encode differences in CPU architecture and OS X
deployment targets in the file name as is done on other platforms,
these continue to be managed by the use of Apple multi-architecture
("fat") files, by the system dynamic loader, and by logic in higher-levels
like sysconfig.get_platform() and pip.