runtime: remove misleading message during crash

The following checkdead message is false positive:

$ go test -race -c runtime
$ ./runtime.test -test.cpu=2 -test.run=TestSmhasherWindowed -test.v
=== RUN TestSmhasherWindowed-2
checkdead: find g 18 in status 1
SIGABRT: abort
PC=0x42bff1

LGTM=rsc
R=golang-codereviews, gobot, rsc
CC=golang-codereviews, iant, khr
https://golang.org/cl/59490046

src/pkg/runtime/proc.c

@@ -2441,8 +2441,14 @@ checkdead(void)
 	run = runtime·sched.mcount - runtime·sched.nmidle - runtime·sched.nmidlelocked - 1;
 	if(run > 0)
 		return;
+	// If we are dying because of a signal caught on an already idle thread,
+	// freezetheworld will cause all running threads to block.
+	// And runtime will essentially enter into deadlock state,
+	// except that there is a thread that will call runtime·exit soon.
+	if(runtime·panicking > 0)
+		return;
 	if(run < 0) {
-		runtime·printf("checkdead: nmidle=%d nmidlelocked=%d mcount=%d\n",
+		runtime·printf("runtime: checkdead: nmidle=%d nmidlelocked=%d mcount=%d\n",
 			runtime·sched.nmidle, runtime·sched.nmidlelocked, runtime·sched.mcount);
 		runtime·throw("checkdead: inconsistent counts");
 	}
@@ -2457,7 +2463,7 @@ checkdead(void)
 			grunning++;
 		else if(s == Grunnable || s == Grunning || s == Gsyscall) {
 			runtime·unlock(&allglock);
-			runtime·printf("checkdead: find g %D in status %d\n", gp->goid, s);
+			runtime·printf("runtime: checkdead: find g %D in status %d\n", gp->goid, s);
 			runtime·throw("checkdead: runnable g");
 		}
 	}