cmd/link: fix test flag

Doesn't cause an error, see #25085.
But we should fix it nonetheless.

Change-Id: I7b6799e0a95475202cacefc3a7f02487e61bfd31
Reviewed-on: https://go-review.googlesource.com/109355Reviewed-by: Heschi Kreinick <heschi@google.com>

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

@@ -819,7 +819,7 @@ func TestRuntimeTypeAttr(t *testing.T) {
 	}
 
 	// Explicitly test external linking, for dsymutil compatility on Darwin.
-	for _, flags := range []string{"-ldflags=linkmode=internal", "-ldflags=-linkmode=external"} {
+	for _, flags := range []string{"-ldflags=-linkmode=internal", "-ldflags=-linkmode=external"} {
 		t.Run("flags="+flags, func(t *testing.T) {
 			if runtime.GOARCH == "ppc64" && strings.Contains(flags, "external") {
 				t.Skip("-linkmode=external not supported on ppc64")