Add target_cpu and target_cpu_only to gclient base config

BUG=807986
R=dpranke

Change-Id: I6b7e94d489a228c5dde23a9b6c7d6f58ce73b180
Reviewed-on: https://chromium-review.googlesource.com/905285Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>

recipes/recipe_modules/gclient/config.py

@@ -35,6 +35,8 @@ def BaseConfig(USE_MIRROR=True, CACHE_DIR=None,
     hooks = List(basestring),
     target_os = Set(basestring),
     target_os_only = Single(bool, empty_val=False, required=False),
+    target_cpu = Set(basestring),
+    target_cpu_only = Single(bool, empty_val=False, required=False),
     cache_dir = Static(cache_dir, hidden=False),
 
     # If supplied, use this as the source root (instead of the first solution's