sync/atomic: update comment for ARM 64-bit atomics
sync/atomic: update comment for ARM 64-bit atomics

They don't work on older chips, just like the x86-32 64-bit atomics.

Update issue 6134

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12880043

rdp/rogerpack2005-golang,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,d0f/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,webfd/go-zh,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof