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

tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang