sync/atomic: add more missing Go prototype
sync/atomic: add more missing Go prototype

Should fix nacl/arm build.

TBR=minux
CC=golang-codereviews
https://codereview.appspot.com/145070043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go