bpo-39936: _aix_support uses _bootsubprocess (GH-18970)

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.