cmd/internal/obj/x86: fix issue19518_test in module mode

Updates #30228

Change-Id: I6a38269f322d906702921b3879ff48c8a96ab511
Reviewed-on: https://go-review.googlesource.com/c/162831
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/obj/x86/issue19518_test.go

@@ -41,6 +41,10 @@ func objdumpOutput(t *testing.T) []byte {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(tmpdir)
+	err = ioutil.WriteFile(filepath.Join(tmpdir, "go.mod"), []byte("module issue19518\n"), 0666)
+	if err != nil {
+		t.Fatal(err)
+	}
 	tmpfile, err := os.Create(filepath.Join(tmpdir, "input.s"))
 	if err != nil {
 		t.Fatal(err)