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
cmd/go: adjust import comment error

Fixes issue 7453.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/127210043
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://codereview.appspot.com/128070045