sync: use atomic.Store in Once.Do

No perf/semantic changes, merely improves code health.
There were several questions as to why Once.Do uses
atomic.CompareAndSwap to do a store.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6208057

src/pkg/sync/once.go

@@ -38,6 +38,6 @@ func (o *Once) Do(f func()) {
 	defer o.m.Unlock()
 	if o.done == 0 {
 		f()
-		atomic.CompareAndSwapUint32(&o.done, 0, 1)
+		atomic.StoreUint32(&o.done, 1)
 	}
 }