Use default number of cores for gclient jobs

Instead of always setting jobs to 1 for ARM (which includes Mac), use
number of cores. Some old boards have not worked well with parallel
sync, but I don't have historical context was going on. My assumption is
that those boards were single-core.

See: https://codereview.chromium.org/10116033

R=aravindvasudev@google.com

Bug: 1410757
Change-Id: I0b7e3363375ba719c73d4d7141e6cab34bf7d32f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4198732
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>

gclient.py

@@ -3355,9 +3355,9 @@ class OptionParser(optparse.OptionParser):
     optparse.OptionParser.__init__(
         self, version='%prog ' + __version__, **kwargs)
 
-    # Some arm boards have issues with parallel sync.
+    # Some old arm boards have issues with parallel sync, so use default cores.
     if platform.machine().startswith('arm'):
-      jobs = 1
+      jobs = gclient_utils.NumLocalCpus()
     else:
       jobs = max(8, gclient_utils.NumLocalCpus())