cmd/ld: support archives larger than 2G

R=golang-codereviews, gobot, rsc
CC=golang-codereviews
https://golang.org/cl/53950043

src/cmd/ld/lib.c

@@ -269,8 +269,8 @@ loadlib(void)
  * look for the next file in an archive.
  * adapted from libmach.
  */
-int
-nextar(Biobuf *bp, int off, struct ar_hdr *a)
+static vlong
+nextar(Biobuf *bp, vlong off, struct ar_hdr *a)
 {
 	int r;
 	int32 arsize;
@@ -300,7 +300,7 @@ nextar(Biobuf *bp, int off, struct ar_hdr *a)
 void
 objfile(char *file, char *pkg)
 {
-	int32 off, l;
+	vlong off, l;
 	Biobuf *f;
 	char magbuf[SARMAG];
 	char pname[150];

src/cmd/ld/lib.h

@@ -241,7 +241,6 @@ void*	mal(uint32 n);
 void	mark(LSym *s);
 void	mywhatsys(void);
 struct ar_hdr;
-int	nextar(Biobuf *bp, int off, struct ar_hdr *a);
 void	objfile(char *file, char *pkg);
 void	patch(void);
 int	pathchar(void);