runtime: make return from main wait for active panic

Arguably if this happens the program is buggy anyway,
but letting the panic continue looks better than interrupting it.
Otherwise things like this are possible, and confusing:

$ go run x.go
panic: $ echo $?
0
$

Fixes #3934.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7322083

src/pkg/runtime/proc.c

@@ -253,6 +253,14 @@ runtime·main(void)
 	main·main();
 	if(raceenabled)
 		runtime·racefini();
+	
+	// Make racy client program work: if panicking on
+	// another goroutine at the same time as main returns,
+	// let the other goroutine finish printing the panic trace.
+	// Once it does, it will exit. See issue 3934.
+	if(runtime·panicking)
+		runtime·park(nil, nil, "panicwait");
+
 	runtime·exit(0);
 	for(;;)
 		*(int32*)runtime·main = 0;