runtime: add missing if(raceenabled)

R=0xe2.0x9a.0x9b, minux.ma, iant, dave
CC=golang-dev
https://golang.org/cl/6654052

src/pkg/runtime/malloc.goc

@@ -693,7 +693,8 @@ runtime·new(Type *typ, uint8 *ret)
 {
 	uint32 flag;
 
-	m->racepc = runtime·getcallerpc(&typ);
+	if(raceenabled)
+		m->racepc = runtime·getcallerpc(&typ);
 	flag = typ->kind&KindNoPointers ? FlagNoPointers : 0;
 	ret = runtime·mallocgc(typ->size, flag, 1, 1);