or1k-tdep.c: Add or1k_push_dummy_code an use it.

 * or1k-tdep.c (or1k_push_dummy_code): New function.
 (or1k_gdbarch_init): Override default behavior and call dummy
 from the stack. Set the push_dummy_code handler to
 or1k_push_dummy_code.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>