8c: only refer to extern register variables in MOVL instructions

R=ken2
https://golang.org/cl/183136

src/cmd/8c/cgen.c

@@ -55,6 +55,12 @@ cgen(Node *n, Node *nn)
 	l = n->left;
 	r = n->right;
 	o = n->op;
+
+	if(n->op == OEXREG || (nn != Z && nn->op == OEXREG)) {
+		gmove(n, nn);
+		return;
+	}
+
 	if(n->addable >= INDEXED) {
 		if(nn == Z) {
 			switch(o) {

src/cmd/8c/sgen.c

@@ -98,6 +98,7 @@ indexshift(Node *n)
  *		(20) * (X) ==> 7	multiplier in indexing
  *		(X,7) + (13,1) ==> 8	adder in indexing (addresses)
  *		(8) ==> &9(OINDEX)	index, almost addressable
+ *		100					extern register
  *
  *	calculate complexity (number of registers)
  */
@@ -125,7 +126,7 @@ xcom(Node *n)
 		break;
 
 	case OEXREG:
-		n->addable = 10;
+		n->addable = 0;
 		break;
 
 	case OREGISTER: