[gclient] Enable parallel sync on ARM Mac by default

I realized that gclient sync doesn't use all the cores on ARM Mac.
This CL changes the default for ARM Mac to use all the available cores.

Change-Id: Id0316e48d2b16a05a397a69bad53172b3d742f4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5256559
Commit-Queue: Junji Watanabe <jwata@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>
Auto-Submit: Junji Watanabe <jwata@google.com>

gclient.py

@@ -4003,7 +4003,8 @@ class OptionParser(optparse.OptionParser):
                                        **kwargs)
 
         # Some arm boards have issues with parallel sync.
-        if platform.machine().startswith('arm'):
+        # ARM Mac should be fine.
+        if sys.platform != 'darwin' and platform.machine().startswith('arm'):
             jobs = 1
         else:
             jobs = max(8, gclient_utils.NumLocalCpus())