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In preparation for the addition of long descriptor page table support,
start moving low-level page table routines into their own file
(s.ShortDesc) so that we can add a corresponding long descriptor
implementation in the future.

* logical_to_physical, MakePageTablesCacheable,
MakePageTablesNonCacheable, AllocateBackingLevel2, AMB_movepagesin_L2PT,
AMB_movecacheablepagesout_L2PT, AMB_moveuncacheablepagesout_L2PT
routines, and PageNumToL2PT macros, all moved to s.ShortDesc with no
changes.
* Add new UpdateL1PTForPageReplacement routine (by splitting some code
out of s.ChangeDyn)