runtime: call atomic.Storeuintptr in noteclear on AIX

The memory might not be synchronized in a thread being woken up after a
semasleep. Using atomic instructions in noteclear function will force
this synchronisation.

Fixes #30189

Change-Id: If7432f29b2a1a56288231822db52f3f8d1d6dbfe
Reviewed-on: https://go-review.googlesource.com/c/go/+/163624
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/lock_sema.go

@@ -122,7 +122,13 @@ func unlock(l *mutex) {
 
 // One-time notifications.
 func noteclear(n *note) {
+	if GOOS == "aix" {
+		// On AIX, semaphores might not synchronize the memory in some
+		// rare cases. See issue #30189.
+		atomic.Storeuintptr(&n.key, 0)
+	} else {
 		n.key = 0
+	}
 }
 
 func notewakeup(n *note) {