FreeBSD: fix static module build broken in 7bb707ffa

param_set_arc_free_target(SYSCTL_HANDLER_ARGS) and
param_set_arc_no_grow_shift(SYSCTL_HANDLER_ARGS) defined in
sysctl_os.c must be made available to arc_os.c.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Martin Matuska <mm@FreeBSD.org>
Closes #13915