cmd/link: skip TestRuntimeTypeDIEs on Plan 9

TestRuntimeTypeDIEs has been added in CL 38350. This
test is failing on Plan 9 because executables don't
have a DWARF symbol table.

Fixes #19944.

Change-Id: I121875bfd5f9f02ed668f8fb0686a0edffa2a99d
Reviewed-on: https://go-review.googlesource.com/40452
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -12,11 +12,17 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"testing"
 )
 
 func TestRuntimeTypeDIEs(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
+
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; no DWARF symbol table in executables")
+	}
+
 	dir, err := ioutil.TempDir("", "TestRuntimeTypeDIEs")
 	if err != nil {
 		t.Fatalf("could not create directory: %v", err)