gc: silence Plan 9 warnings

R=golang-dev
CC=golang-dev, rsc
https://golang.org/cl/4975055

src/cmd/gc/sinit.c

@@ -1260,6 +1260,8 @@ addvalue(InitPlan *p, vlong xoffset, Node *key, Node *n)
 	InitPlan *q;
 	InitEntry *e;
 
+	USED(key);
+
 	// special case: zero can be dropped entirely
 	if(iszero(n)) {
 		p->zero += n->type->width;

src/cmd/gc/typecheck.c

@@ -1031,7 +1031,6 @@ reswitch:
 		}
 		n->left = args->n;
 		n->right = args->next->n;
-		args = nil;
 		n->list = nil;
 		n->type = types[TINT];
 		typecheck(&n->left, Erv);