Also remove UnsafeAcquire from UseScratchRegisterScope, which was only used for
these stubs.

Bug: 
Change-Id: Ia8648e53f1165ae489c0475dbb2d10c6978e4e84
Reviewed-on: https://chromium-review.googlesource.com/576181Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Georgia Kouveli <georgia.kouveli@arm.com>
Cr-Commit-Position: refs/heads/master@{#46761}