MFC r273701, r274556

MFC r273701, r274556
  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.)

git-svn-id: svn://svn.freebsd.org/base/stable/10@276546 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f