Remove SetStackPointer function and update assembler tests.

Bug: v8:6644
Change-Id: I5ea26e57b848d56433c84d8eea22d7e9b761e6bb
Reviewed-on: https://chromium-review.googlesource.com/864147Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#50571}