This CL adds target_cpu and target_cpu_only variables.  The logic is:

* If target_cpu is defined in .gclient, use it.  Otherwise, initialize it as an
  empty list.
* If not target_cpu_only, add the host arch to the list too.

detect_host_arch.py is copied directly from the Chromium repo.

BUG=807986
R=dpranke

Change-Id: I27621cbc81ad6a844648525863b92ffdd3b1d03a
Reviewed-on: https://chromium-review.googlesource.com/902441
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>