[gm] Adjust the script to support mips64el

R=jkummerow@chromium.org

Change-Id: I6fc3817410df4f070675051397a30cc1b0ca7dfe
Reviewed-on: https://chromium-review.googlesource.com/488030Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44929}

tools/dev/gm.py

@@ -176,7 +176,7 @@ class Config(object):
 
   def GetTargetCpu(self):
     cpu = "x86"
-    if self.arch.endswith("64") or self.arch == "s390x":
+    if "64" in self.arch or self.arch == "s390x":
       cpu = "x64"
     return "target_cpu = \"%s\"" % cpu