- Properly detect whether the args.gn config actually wants goma.
- Read out cpu count dynamically.

NOTRY=true

Bug: 
Change-Id: I7a687e873ef0b009ab6eaada384378d23e1dbb1e
Reviewed-on: https://chromium-review.googlesource.com/558085
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46362}