Merge some stack operations to work on an even number of registers, adding
a padding register where necessary. Some lightly-used macro assembler
functions are inlined, to make pairing registers easier. Not all merges
create an even number of register arguments yet.

This is a step towards aligning the stack pointer to 16-bytes.

Bug: v8:6644
Change-Id: I995510cf91fa1f7af659a8d9b83acf7857787100
Reviewed-on: https://chromium-review.googlesource.com/654607
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47975}