We were attempting to assemble code into the MacroAssembler buffer after
executing it, without resetting the permissions. As a result, tests that
are using START/END multiple times were failing.

Change-Id: Id84c6a07212a869f98edbd33d86ff70ee6c819db
Reviewed-on: https://chromium-review.googlesource.com/939388Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georgia Kouveli <georgia.kouveli@arm.com>
Cr-Commit-Position: refs/heads/master@{#51901}