cmd/internal/obj/arm64, cmd/link: use two instructions rather than three for loads from memory

Reduces size of godoc .text section by about 75k (or 1.4%).

Change-Id: I65850aa569aefbddd6cb07c6ae1addcc39cab6a5
Reviewed-on: https://go-review.googlesource.com/13993


Reviewed-by: Russ Cox <rsc@golang.org>