On AArch64, we can't push arguments one by one because sp must always be 16 byte aligned.
This patch adds a LiftoffStackSlots class which holds everything which has to be pushed.
This way, on AArch64, we will be able to reserve the needed space and eventually add some padding. Then, all pushes will be converted to stores into this reserved space.

Bug: v8:6600

Change-Id: I17480fb841f16e07356b35326c59e3c7f03ed012
Reviewed-on: https://chromium-review.googlesource.com/1023977
Commit-Queue: Vincent Belliard <vincent.belliard@arm.com>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52746}