cmd/objdump: skip TestDisasm* on s390x

The disassembler is not yet implemented on s390x.

Updates #15255.

Change-Id: Ibab319c8c087b1a619baa1529398305c1e721877
Reviewed-on: https://go-review.googlesource.com/21894Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/objdump/objdump_test.go

@@ -107,6 +107,8 @@ func TestDisasm(t *testing.T) {
 		t.Skipf("skipping on %s, issue 10106", runtime.GOARCH)
 	case "mips64", "mips64le":
 		t.Skipf("skipping on %s, issue 12559", runtime.GOARCH)
+	case "s390x":
+		t.Skipf("skipping on %s, issue 15255", runtime.GOARCH)
 	}
 	testDisasm(t)
 }
@@ -123,6 +125,8 @@ func TestDisasmExtld(t *testing.T) {
 		t.Skipf("skipping on %s, issue 10106", runtime.GOARCH)
 	case "mips64", "mips64le":
 		t.Skipf("skipping on %s, issue 12559 and 12560", runtime.GOARCH)
+	case "s390x":
+		t.Skipf("skipping on %s, issue 15255", runtime.GOARCH)
 	}
 	// TODO(jsing): Reenable once openbsd/arm has external linking support.
 	if runtime.GOOS == "openbsd" && runtime.GOARCH == "arm" {