AIX: sys.platform always returns 'aix' instead of 'aix6' or 'aix7'.

sys.platform doesn't contain major version of aix.
Always return 'aix', instead of 'aix6' or 'aix7'.

R=gavinmak@google.com, kimstephanie@google.com

Change-Id: I6d2075be808503731996df7bf17f23527ebdf445
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5999752
Commit-Queue: Scott Lee <ddoman@chromium.org>
Reviewed-by: Scott Lee <ddoman@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>

download_from_google_storage.py

@@ -33,8 +33,7 @@ PLATFORM_MAPPING = {
     'linux': 'linux',  # Python 3.3+.
     'linux2': 'linux',  # Python < 3.3 uses "linux2" / "linux3".
     'win32': 'win',
-    'aix6': 'aix',
-    'aix7': 'aix',
+    'aix': 'aix',  # Python 3.8+
     'zos': 'zos',
 }