loadustp(): Optimize for the overwhelmingly common case of "not the HP MMU".
Rather than converting the level 1 table address to a page number before
calling loadustp() only to have loadustp() convert it back to an address
for the '851, '030, '040, and '060, instead pass the address and convert
to a page number only in the case of the HP MMU.

This is a wash on HP MMU machines (9000/320 and 9000/350), and saves at
least 4 instructions (2x moveq + 2x lsXl) on every context switch on
everything else.

(thorpej)