The FuzzAssembleSwap test was failing with "--arm-arch=armv6". The reason was
that we were assuming we always had 2 D registers available as scratches, which
isn't the case if VFP32DREGS isn't enabled.

Change-Id: Ie97b48fd36fcbdbc6e137412c148a0bf58b498a6
Reviewed-on: https://chromium-review.googlesource.com/957733
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52132}