Rename csp to sp and remove support for the stack pointer abstraction and
switching stack pointers.

Bug: v8:6644
Change-Id: I616633aabc1cee9926249fe95ce6c37ed6544fe3
Reviewed-on: https://chromium-review.googlesource.com/870870Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#50687}