Add PPC/s390 configs to mb_config and BUILD.gn. Also
use a script to get host byte ordering.

R=machenbach@chromium.org, dpranke@chromium.org
BUG=
LOG=N

Review-Url: https://codereview.chromium.org/2736993004
Cr-Commit-Position: refs/heads/master@{#43751}