Collect code coverage by compiling for one or more target architectures
and then running tests, in the same directory. This way, gcov aggregates
results.

Bug: 
Change-Id: I3bf05416c535c0c566e48d4e73adc4eb49ba2793
Reviewed-on: https://chromium-review.googlesource.com/527522
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46110}