[dev.power64] 5g: fix etype and width of itable Addrs

For OITAB nodes, 5g's naddr was setting the wrong etype and
failing to set the width of the resulting Addr.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/171220043

src/cmd/5g/gsubr.c

@@ -1353,9 +1353,10 @@ naddr(Node *n, Addr *a, int canemitcode)
 	case OITAB:
 		// itable of interface value
 		naddr(n->left, a, canemitcode);
-		a->etype = TINT32;
+		a->etype = simtype[tptr];
 		if(a->type == D_CONST && a->offset == 0)
 			break;	// len(nil)
+		a->width = widthptr;
 		break;
 
 	case OSPTR: