cmd/compile: skip TestEmptyDwarfRanges on Plan 9

TestEmptyDwarfRanges has been added in CL 94816.
This test is failing on Plan 9 because executables
don't have a DWARF symbol table.

Fixes #24226.

Change-Id: Iff7e34b8c2703a2f19ee8087a4d64d0bb98496cd
Reviewed-on: https://go-review.googlesource.com/98275Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/scope_test.go

@@ -447,6 +447,10 @@ func gobuild(t *testing.T, dir string, optimized bool, testfile []testline) (str
 func TestEmptyDwarfRanges(t *testing.T) {
 	testenv.MustHaveGoRun(t)
 
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; no DWARF symbol table in executables")
+	}
+
 	dir, err := ioutil.TempDir("", "TestEmptyDwarfRanges")
 	if err != nil {
 		t.Fatalf("could not create directory: %v", err)