runtime: disable TestCgoCallbackGC on FreeBSD

The trybot flakes are a nuisance.

Updates #16396

Change-Id: I8202adb554391676ba82bca44d784c6a81bf2085
Reviewed-on: https://go-review.googlesource.com/27313
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/crash_cgo_test.go

@@ -42,8 +42,11 @@ func TestCgoTraceback(t *testing.T) {
 }
 
 func TestCgoCallbackGC(t *testing.T) {
-	if runtime.GOOS == "plan9" || runtime.GOOS == "windows" {
+	switch runtime.GOOS {
+	case "plan9", "windows":
 		t.Skipf("no pthreads on %s", runtime.GOOS)
+	case "freebsd":
+		testenv.SkipFlaky(t, 16396)
 	}
 	if testing.Short() {
 		switch {