physical addresses.
o) Set a local maxmem in sb_machdep.c to avoid trying to use pages over 2^64
under 32-bit ABIs. Our pmap needs corrected to use vm_paddr_t consistently,
then we can make vm_paddr_t 64-bit under 32-bit ABIs and add code in pmap
to limit phys_avail by the maximum PFN that a 32-bit PTE can hold.
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