runtime: copy netpoll code from Go 1.7 runtime

Reviewed-on: https://go-review.googlesource.com/31325

From-SVN: r241307

libgo/go/runtime/netpoll_nacl.go

@@ -0,0 +1,26 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Fake network poller for NaCl.
+// Should never be used, because NaCl network connections do not honor "SetNonblock".
+
+package runtime
+
+func netpollinit() {
+}
+
+func netpollopen(fd uintptr, pd *pollDesc) int32 {
+	return 0
+}
+
+func netpollclose(fd uintptr) int32 {
+	return 0
+}
+
+func netpollarm(pd *pollDesc, mode int) {
+}
+
+func netpoll(block bool) *g {
+	return nil
+}