The Linux arc_os.c carries userspace and kernel code, with very little
overlap between the two. This lifts the userspace parts out into a
separate arc_os.c for libzpool and removes it from the Linux side.
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/Closes#16492
