Streamlines some arguments.
Use a common subdirectory in out/.
Show process name when logging.
Clean up environment variable handling.

Bug: angleproject:5247
Change-Id: I85eb5b8194a64f90691c4d03db3ee65c5e9b2313
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2497560Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>