Introducing a new status-file variable: mips_arch_variant. With this
variable, in status files will be possible to define selections which
are based on MIPS architecture revisions/variants.

TEST=
BUG=

Change-Id: Ifd682552db2f26be4e56dc94ad50bed063ff14c5
Reviewed-on: https://chromium-review.googlesource.com/941212Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#51647}