Add MSA detect to unittest.
Change macro to disable DSPR2 code to LIBYUV_DISABLE_DSPR2

BUG=libyuv:634
TEST=try bots

Change-Id: I9e0aa2452204fc529bb6f9e6fd93c4e1c379bba6
Reviewed-on: https://chromium-review.googlesource.com/433463Reviewed-by: Henrik Kjellander <kjellander@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@google.com>