Correction to GuessOS operation in PASE environment

Change-Id: I0ca51abf2468f773986645fec3cb98559bca5bbe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2545143Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Milad Fa <mfarazma@redhat.com>
Commit-Queue: Vasili Skurydzin <vasili.skurydzin@ibm.com>
Cr-Commit-Position: refs/heads/master@{#71295}

tools/testrunner/local/utils.py

@@ -90,7 +90,8 @@ def GuessOS():
     return 'solaris'
   elif system == 'NetBSD':
     return 'netbsd'
-  elif system == 'AIX':
+  elif system in ['AIX', 'OS400']:
+    # OS400 runs an AIX emulator called PASE
     return 'aix'
   else:
     return None