s390x: add s390x disassembler support for the plan9

Change-Id: I0eb9a10535175bb70dbab5737c4e02e68fd44c94
Reviewed-on: https://go-review.googlesource.com/c/arch/+/620475
Reviewed-by: Cherry Mui <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: Vishwanatha HD <[email protected]>

Author: srinivas-pokala

s390x/s390xasm/decode_test.go

@@ -75,6 +75,8 @@ func decode(data []byte, t *testing.T, filename string) {
 			switch syntax {
 			case "gnu":
 				out = GNUSyntax(inst, pc)
+			case "plan9":
+				out = GoSyntax(inst, pc, nil)
 			default:
 				t.Errorf("unknown syntax %q", syntax)
 				continue