doc/go1.4: mention pc-relative addressing on amd64
doc/go1.4: mention pc-relative addressing on amd64

CC=golang-codereviews
https://codereview.appspot.com/124580043

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