cmd: stop looking for __.(GO)?SYMDEF entries in archives

The Go toolchain stopped creating them before Go 1.3, so no point in
worrying about them today.

History:

- Git commit 250a091 added cmd/ar, which wrote Plan 9 __.SYMDEF
entries into archive files.

- golang.org/cl/6500117 renamed __.SYMDEF to __.GOSYMDEF.  (Notably,
the commit message suggests users need to use Go nm to read symbols,
but even back then the toolchain did nothing with __.(GO)?SYMDEF files
except skip over them.)

- golang.org/cl/42880043 added the -pack flag to cmd/gc to directly
produce archives by the Go compiler, and did not write __.GOSYMDEF
entries.

- golang.org/cl/52310044 rewrote cmd/pack in Go, and removed support
for producing __.GOSYMDEF entries.

Change-Id: I255edf40d0d3690e3447e488039fcdef73c6d6b1
Reviewed-on: https://go-review.googlesource.com/19924
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/link/internal/ld/lib.go

@@ -255,10 +255,7 @@ var coutbuf struct {
 	f *os.File
 }
 
-const (
-	symname = "__.GOSYMDEF"
-	pkgname = "__.PKGDEF"
-)
+const pkgname = "__.PKGDEF"
 
 var (
 	// Set if we see an object compiled by the host compiler that is not
@@ -781,7 +778,7 @@ func objfile(lib *Library) {
 		return
 	}
 
-	/* skip over optional __.GOSYMDEF and process __.PKGDEF */
+	/* process __.PKGDEF */
 	off := obj.Boffset(f)
 
 	var arhdr ArHdr
@@ -792,15 +789,6 @@ func objfile(lib *Library) {
 		goto out
 	}
 
-	if strings.HasPrefix(arhdr.name, symname) {
-		off += l
-		l = nextar(f, off, &arhdr)
-		if l <= 0 {
-			Diag("%s: short read on archive file symbol header", lib.File)
-			goto out
-		}
-	}
-
 	if !strings.HasPrefix(arhdr.name, pkgname) {
 		Diag("%s: cannot find package header", lib.File)
 		goto out
@@ -829,7 +817,7 @@ func objfile(lib *Library) {
 	 * the individual symbols that are unused.
 	 *
 	 * loading every object will also make it possible to
-	 * load foreign objects not referenced by __.GOSYMDEF.
+	 * load foreign objects not referenced by __.PKGDEF.
 	 */
 	for {
 		l = nextar(f, off, &arhdr)