Switch stack pointer to using csp directly, making jssp redundant.

Bug: v8:6644
Change-Id: I8e38eda50d56a25161b187c0a033608dd9f90239
Reviewed-on: https://chromium-review.googlesource.com/860097Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#50487}