The MathPowInternal builtin is now only called directly from the
code-generator. Also, this patch takes the opportunity to cleanup the builtin
for arm and arm64 a little.

Change-Id: If53edcecd42b227ef74ee817bc5d7fbbcea8b0e2
Reviewed-on: https://chromium-review.googlesource.com/1076127Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#53442}