runtime: avoid using address as constant in amd64 assembly
runtime: avoid using address as constant in amd64 assembly

This allows changing the addressing mode for constant
global addresses to use pc-relative addressing.

LGTM=rminnich, iant
R=golang-codereviews, rminnich, iant
CC=golang-codereviews
https://codereview.appspot.com/129830043

wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old