Remove remaining references to jssp, and return register to the allocator.

Bug: v8:6644
Change-Id: Ia6938e6c9548cd45d8c9c12032920b32d3da3c4c
Reviewed-on: https://chromium-review.googlesource.com/866747Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#50613}