runtime: say when a goroutine is locked to OS thread

Say when a goroutine is locked to OS thread in crash reports
and goroutine profiles.
It can be useful to understand what goroutines consume OS threads
(syscall and locked), e.g. if you forget to call UnlockOSThread
or leak locked goroutines.

R=golang-codereviews
CC=golang-codereviews, rsc
https://golang.org/cl/94170043

src/pkg/runtime/proc.c

@@ -297,10 +297,12 @@ runtime·goroutineheader(G *gp)
 	if((gp->status == Gwaiting || gp->status == Gsyscall) && gp->waitsince != 0)
 		waitfor = (runtime·nanotime() - gp->waitsince) / (60LL*1000*1000*1000);
 
-	if(waitfor < 1)
-		runtime·printf("goroutine %D [%s]:\n", gp->goid, status);
-	else
-		runtime·printf("goroutine %D [%s, %D minutes]:\n", gp->goid, status, waitfor);
+	runtime·printf("goroutine %D [%s", gp->goid, status);
+	if(waitfor >= 1)
+		runtime·printf(", %D minutes", waitfor);
+	if(gp->lockedm != nil)
+		runtime·printf(", locked to thread");
+	runtime·printf("]:\n");
 }
 
 void