runtime: better debug output for inconsistent Note

Update #5139.
Double wakeup on Note was reported several times,
but no reliable reproducer.
There also was a strange report about weird value of epoll fd.
Maybe it's corruption of global data...

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12182043

src/pkg/runtime/lock_futex.c

@@ -118,8 +118,13 @@ runtime·noteclear(Note *n)
 void
 runtime·notewakeup(Note *n)
 {
-	if(runtime·xchg((uint32*)&n->key, 1))
+	uint32 old;
+
+	old = runtime·xchg((uint32*)&n->key, 1);
+	if(old != 0) {
+		runtime·printf("notewakeup - double wakeup (%d)\n", old);
 		runtime·throw("notewakeup - double wakeup");
+	}
 	runtime·futexwakeup((uint32*)&n->key, 1);
 }