FreeBSD: Remove some code duplication in sysctl_os.c

Drop unnecessary redefinition's of several arcstat values.
Put missing extern declaration of arc_no_grow_shift in arc_impl.h.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10609