By the time that pmap_init() runs, vm_phys_segs[] has been initialized.
Obtaining the end of memory address from vm_phys_segs[] is a little
easier than obtaining it from phys_avail[].

Enable the use of VM_PHYSSEG_SPARSE on amd64 and i386, making it the
default on i386 PAE.  (The use of VM_PHYSSEG_SPARSE on i386 PAE saves
us some precious kernel virtual address space that would have been
wasted on unused vm_page structures.)