Set the current pmap in macppc's pmap_activate

Set the current pmap in macppc's pmap_activate

This fixes a possible freeze in execve(2).  It sometimes froze when a
dual-cpu macppc started daemons during boot.  There is a chance that
uvm_map.c uvmspace_exec sees ovm->vm_refcnt != 1 and switches curproc
to a new pmap.  If this happened, then execve froze by trying to
copyout to the wrong pmap; curpcb->pcb_pm was old.  Fix by setting
pointers when uvmspace_exec calls pmap_activate.

ok miod@