Restore compatibility with aix_support on Python 3.8 and earlier. Fixes pypa/setuptools#2358.

distutils/py38compat.py

+def aix_platform(osname, version, release):
+	try:
+        import _aix_support
+		return _aix_support.aix_platform()
+	except ImportError:
+		pass
+    return "%s-%s.%s" % (osname, version, release)

distutils/util.py

@@ -79,8 +79,8 @@ def get_host_platform():
             machine += ".%s" % bitness[sys.maxsize]
         # fall through to standard osname-release-machine representation
     elif osname[:3] == "aix":
-        from _aix_support import aix_platform
-        return aix_platform()
+        from .py38compat import aix_platform
+        return aix_platform(osname, version, release)
     elif osname[:6] == "cygwin":
         osname = "cygwin"
         rel_re = re.compile (r'[\d.]+', re.ASCII)