cmd/6c, cmd/6g: avoid address-as-constant in amd64 instructions

This allows implementing address-of-global
as a pc-relative address instead of as a
32-bit integer constant.

LGTM=rminnich, iant
R=golang-codereviews, rminnich, iant
CC=golang-codereviews
https://golang.org/cl/128070045
3 files changed