In addition, this makes the arm64 prologue code generation much closer
to the pattern used on other platforms.

This passes toolstash -cmp with one exception: assembly functions that
were declared with a frame size of -8 used to record
locals=0xfffffffffffffff8 in the object file and now record
locals=0x0. This doesn't affect anything.

Change-Id: I0d15e81770e54222ae329ce4496da06016736770
Reviewed-on: https://go-review.googlesource.com/92040
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>