runtime: eliminate excessive notewakeup calls in timers

If the timer goroutine is wakeup by timeout,
other goroutines will still notewakeup because sleeping is still set.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/12763043

src/pkg/runtime/time.goc

@@ -175,6 +175,7 @@ timerproc(void)
 
 	for(;;) {
 		runtime·lock(&timers);
+		timers.sleeping = false;
 		now = runtime·nanotime();
 		for(;;) {
 			if(timers.len == 0) {