FIXUP: CHROMIUM: Add process mgmt security policies to chromiumos LSM

chromiumos LSM no longer builds with most architectures due to its use
of symbols and functions not available on all architectures. Restrict
to x86_64 and arm64 since thos are the only tested and supported
architectures, and there are presumably no plans to ever upstream this
code.

BUG=chromium:845640, chromium:649417
TEST=Build non-standard architectures

Change-Id: I3bcf9ba4c6d5f75f85e3b26b7c02c561411c3fb0
Signed-off-by: Guenter Roeck <groeck@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1298701


Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Reviewed-by: Dmitry Torokhov <dtor@chromium.org>