bpo-38021: Modify AIX platform_tag so it covers PEP 425 needs (GH-17303)

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.

util.py

@@ -79,7 +79,8 @@ def get_host_platform():
             machine += ".%s" % bitness[sys.maxsize]
         # fall through to standard osname-release-machine representation
     elif osname[:3] == "aix":
-        return "%s-%s.%s" % (osname, version, release)
+        from _aix_support import aix_platform
+        return aix_platform()
     elif osname[:6] == "cygwin":
         osname = "cygwin"
         rel_re = re.compile (r'[\d.]+', re.ASCII)