AIX: Fix _aix_support module when the subprocess is not available,
when building Python from scratch. It now uses new private
_bootsubprocess module, rather than having two implementations
depending if subprocess is available or not. So
_aix_support.aix_platform() result is now the same if subprocess is
available or not.
Provides a richer platform tag for AIX that we expect to be sufficient for PEP 425
binary distribution identification. Any backports to earlier Python versions will be
handled via setuptools.
Patch by Michael Felt.