runtime: print stack trace when "panic during panic"

Fixes bug 7145

R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/53970043

src/pkg/runtime/panic.c

@@ -371,7 +371,8 @@ runtime·startpanic(void)
 		m->mcache = runtime·allocmcache();
 	if(m->dying) {
 		runtime·printf("panic during panic\n");
-		runtime·exit(3);
+		runtime·dopanic(0);
+		runtime·exit(3); // not reached
 	}
 	m->dying = 1;
 	if(g != nil)