This is mainly about clarifying the frame contents for each call type, though
assertions on the even length of Push/PopRegList() are also added.

Bug: v8:6644
Change-Id: I4dfe6ac2603e2bab895758e85d8dfe06770d8987
Reviewed-on: https://chromium-review.googlesource.com/672844Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#48116}