runtime: don't run TestEnsureDropM on windows or plan9

This is testing code in asm_GOARCH.s, so it's not necessary to run the
test on systems where it doesn't build.

Fixes #13991.

Change-Id: Ia7a2d3a34b32e6987dc67428c1e09e63baf0518a
Reviewed-on: https://go-review.googlesource.com/18707Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/crash_cgo_test.go

@@ -137,6 +137,10 @@ func TestCgoExecSignalMask(t *testing.T) {
 
 func TestEnsureDropM(t *testing.T) {
 	// Test for issue 13881.
+	switch runtime.GOOS {
+	case "windows", "plan9":
+		t.Skipf("skipping dropm test on %s", runtime.GOOS)
+	}
 	got := runTestProg(t, "testprogcgo", "EnsureDropM")
 	want := "OK\n"
 	if got != want {

src/runtime/testdata/testprogcgo/dropm.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
+
 // Test that a sequence of callbacks from C to Go get the same m.
 // This failed to be true on arm and arm64, which was the root cause
 // of issue 13881.