cmd/objdump: fix and re-enable arm test

Fixes #9021

Change-Id: Id4465857c765342aa8c4b5bc2fdc06754a290025
Reviewed-on: https://go-review.googlesource.com/67530Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/objdump/objdump_test.go

@@ -63,7 +63,7 @@ var x86Need = []string{
 }
 
 var armNeed = []string{
-	//"B.LS main.main(SB)", // TODO(rsc): restore; golang.org/issue/9021
+	"B main.main(SB)",
 	"BL main.Println(SB)",
 	"RET",
 }