[test] Run benchmarks less parallel on ODROIDs

This is to prevent errors like:
https://ci.chromium.org/p/v8/builders/luci.v8.ci/V8%20Arm/8792

The error code suggests the OS killed the tests due to OOM. The time it took
suggests OS paging. Fewer tests in parallel should mitigate this.

NOTRY=true

Change-Id: I847058cfb02a9a36795581df47760d921d695141
Reviewed-on: https://chromium-review.googlesource.com/c/1333674
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57486}

infra/testing/builders.pyl

@@ -1057,7 +1057,9 @@
           'cores': '8',
           'os': 'Ubuntu-16.04',
           'cpu': 'armv7l-32-ODROID-XU4',
-        }
+        },
+        # Less parallelism to prevent OOMs in benchmarks.
+        'test_args': ['-j4'],
       },
       {
         'name': 'optimize_for_size',