cmd/6c, cmd/6g: avoid address-as-constant in amd64 instructions

This allows implementing address-of-global
as a pc-relative address instead of as a
32-bit integer constant.

LGTM=rminnich, iant
R=golang-codereviews, rminnich, iant
CC=golang-codereviews
https://golang.org/cl/128070045

src/cmd/6c/sgen.c

@@ -124,10 +124,7 @@ xcom(Node *n)
 		break;
 
 	case ONAME:
-		if(flag_largemodel)
-			n->addable = 9;
-		else
-			n->addable = 10;
+		n->addable = 9;
 		if(n->class == CPARAM || n->class == CAUTO)
 			n->addable = 11;
 		break;

src/cmd/6g/cgen.c

@@ -752,12 +752,7 @@ agenr(Node *n, Node *a, Node *res)
 			regalloc(&n3, types[tptr], res);
 			p1 = gins(ALEAQ, N, &n3);
 			datastring(nl->val.u.sval->s, nl->val.u.sval->len, &p1->from);
-			if(flag_largemodel) {
-				gins(AADDQ, &n2, &n3);
-			} else {
-				p1->from.scale = 1;
-				p1->from.index = n2.val.u.reg;
-			}
+			gins(AADDQ, &n2, &n3);
 			goto indexdone;
 		}
 

src/cmd/6g/gsubr.c

@@ -598,11 +598,8 @@ ismem(Node *n)
 	case ONAME:
 	case OPARAM:
 	case OCLOSUREVAR:
-		return 1;
 	case OADDR:
-		if(flag_largemodel)
-			return 1;
-		break;
+		return 1;
 	}
 	return 0;
 }