Refactor ZFS ARC reclaim checks and limits

Remove previously added kmem methods in favour of defines which
allow diff minimisation between upstream code base.

Rebalance ARC free target to be vm_pageout_wakeup_thresh by default
which eliminates issue where ARC gets minimised instead of balancing
with VM pageout. The restores the target point prior to r270759.

Bring in missing upstream only changes which move unused code to
further eliminate code differences.

Add additional DTRACE probe to aid monitoring of ARC behaviour.

Enable upstream i386 code paths on platforms which don't define
UMA_MD_SMALL_ALLOC.

Fix mixture of byte an page values in arc_memory_throttle i386 code
path value assignment of available_memory.

PR: 187594
Review: D702
Reviewed by: avg
MFC after: 1 week
X-MFC-With: r270759 & r270861
Sponsored by: Multiplay