locore calls to go via a locore-entry jumptable.
Use mips_btop(), mips_round_page, mips_trunc_seg() instead
of pmax_btop(), pmax_round_page, pmax_trunc_seg().
Add Per's software-readonly-bit mechanism, since the r2000 and r4000
hardware TLB entries are very different, and the r4k has no space for
software bits in TLB entries. That is, this pmap code still won't work
on r4000 machines. Some other solution, like another jump table for
clients of the pmap code, is necessary.
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