For mips, if 'mips_arch_variant=="r6"' and if 'mips_use_msa' flag is set
to 'true', then test-run-wasm-simd tests won't be skipped for mips. It
will also force 'MIPS_SIMD' bit in CpuFeatures to be set.
ARM processors are assumed to support SIMD.

Change-Id: Iea668b97ef995ca4949ddbf2ffc734aad89d3aa3
Reviewed-on: https://chromium-review.googlesource.com/868430Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#50981}