cmd/go: skip flaky TestCgoConsistentResults on FreeBSD

FreeBSD 11 or above uses clang-3.6 or higher by default.

Updates #15405.

Change-Id: If49ce298130165f9e1525c7fd0fd5aa39099ad53
Reviewed-on: https://go-review.googlesource.com/34675Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -3357,9 +3357,11 @@ func TestCgoConsistentResults(t *testing.T) {
 	if !canCgo {
 		t.Skip("skipping because cgo not enabled")
 	}
-	if runtime.GOOS == "solaris" {
-		// See https://golang.org/issue/13247
-		t.Skip("skipping because Solaris builds are known to be inconsistent; see #13247")
+	switch runtime.GOOS {
+	case "freebsd":
+		testenv.SkipFlaky(t, 15405)
+	case "solaris":
+		testenv.SkipFlaky(t, 13247)
 	}
 
 	tg := testgo(t)