runtime/cgo: fix netbsd build

Copy over freebsd.c to netbsd.c.

Fixes #3823.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6398045

src/pkg/runtime/cgo/netbsd.c

+// Copyright 2010 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.
+
+// Supply environ and __progname, because we don't
+// link against the standard NetBSD crt0.o and the
+// libc dynamic library needs them.
+
+char *environ[1];
+char *__progname;
+
+#pragma dynexport environ environ
+#pragma dynexport __progname __progname