Add a SlotAddress macro assembler function to abstract obtaining the address of
an sp-relative double word slot. Then, merge some stack accesses in builtins,
generally changing from push to claim and poke.

Bug: v8:6644
Change-Id: Id7a9f6e276a038fbffd8e2bbaa21f51855f057f1
Reviewed-on: https://chromium-review.googlesource.com/723465
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48685}