This enhances to use the native parallel compile extensions if
available.

Bug: 873724

Change-Id: I0aaed314accd75e1bfa596b322225b56d729d3a6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1475234
Commit-Queue: Jie A Chen <jie.a.chen@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>