add and test stack load/store

Change-Id: Ie0d29e31bd8c156ecd46cd658b5a4c53d8d2e11d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/282115
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed