libc: stub getgroups and setgroups

Roman says:

  getgroups(2) needs to be stubbed This is required by
  jre/lib/amd64/libjaas_unix.so and should be pretty easy to do.

Fixes #349.

Reported-by: Roman Shaposhnik <roman@shaposhnik.org>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>