cmd/ld: remove remaining occurrence of isobj.

Fixes cgo builds.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/7784044

src/cmd/ld/lib.h

@@ -143,7 +143,6 @@ EXTERN	int	nldflag;
 EXTERN	char**	ldflag;
 EXTERN	int	havedynamic;
 EXTERN	int	iscgo;
-EXTERN	int	isobj;
 EXTERN	int	elfglobalsymndx;
 EXTERN	int	flag_race;
 EXTERN	int flag_shared;

src/cmd/ld/symtab.c

@@ -136,7 +136,7 @@ putelfsym(Sym *x, char *s, int t, vlong addr, vlong size, int ver, Sym *go)
 		return;
 
 	off = putelfstr(s);
-	if(isobj)
+	if(linkmode == LinkExternal)
 		addr -= xo->sect->vaddr;
 	putelfsyment(off, addr, size, (bind<<4)|(type&0xf), xo->sect->elfsect->shnum, (x->type & SHIDDEN) ? 2 : 0);
 	x->elfsym = numelfsym++;