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

sanjosh/sanjos100-tipc,bryanxu/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,d0f/go-zh,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc