Turn sp_tlb_flush_{ctx,pte} into function pointers, and pick one out of three

Turn sp_tlb_flush_{ctx,pte} into function pointers, and pick one out of three
flavours: pre-usIII, usIII, and sun4v.

This allows us to get rid of the HORRID_III_HACK define in locore and switch
pre-usIII systems to the older, slightly simpler, code for these routines.

ok claudio@ kettenis@