runtime: prepare for M's running w/o mcache

Can not happen ATM. In preparation for the new scheduler.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7388043

src/pkg/runtime/panic.c

@@ -382,6 +382,8 @@ nomatch:
 void
 runtime·startpanic(void)
 {
+	if(m->mcache == nil)  // can happen if called from signal handler or throw
+		m->mcache = runtime·allocmcache();
 	if(m->dying) {
 		runtime·printf("panic during panic\n");
 		runtime·exit(3);