sync/atomic: trigger paging fault early on linux/arm so that we don't need to traceback through __kuser_cmpxchg
sync/atomic: trigger paging fault early on linux/arm
so that we don't need to traceback through __kuser_cmpxchg

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12869043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang