Fix a problem where kernel text could become unmapped when clearing out all

Fix a problem where kernel text could become unmapped when clearing out all
the user mappings from the tlb due to the context numbers rolling over.  The
store to the internal mmu register must be followed by a membar #Sync before
much else happens to "avoid data corruption", so we use special inlines which
both disable interrupts and ensure that the compiler will not insert extra
instructions between the two.  Also, load the tte tag and check if the context
is nucleus context, rather than relying on the priviledged bit which doesn't
actually serve any purpose in our design, and check the lock bit too for
sanity.