Fix arm unit tests.

Registers qemu for the arm architecture, and bind mounts the source tree
into the build root for access.

BUG=chromium:304400
TEST=cros_run_unit_tests --board daisy-drm for various common.mk projects

Change-Id: I7bf3ef0c0495f566c27eaf9bc3e65f69311b8d82
Reviewed-on: https://chromium-review.googlesource.com/173562
Reviewed-by: Albert Chaulk <achaulk@chromium.org>
Tested-by: Albert Chaulk <achaulk@chromium.org>
Commit-Queue: Albert Chaulk <achaulk@chromium.org>