runtime: don't run TestCgoNumGoroutine on Windows or Plan 9

The test requires pthreads.

Fixes #20666.

Change-Id: Icb2400250a80cdad6680cd1ef6c18ef7343d5e29
Reviewed-on: https://go-review.googlesource.com/45701
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/crash_cgo_test.go

@@ -397,6 +397,10 @@ func TestRaceSignal(t *testing.T) {
 }
 
 func TestCgoNumGoroutine(t *testing.T) {
+	switch runtime.GOOS {
+	case "windows", "plan9":
+		t.Skipf("skipping numgoroutine test on %s", runtime.GOOS)
+	}
 	t.Parallel()
 	got := runTestProg(t, "testprogcgo", "NumGoroutine")
 	want := "OK\n"

src/runtime/testdata/testprogcgo/numgoroutine.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !plan9,!windows
+
 package main
 
 /*