Once more into the breach, dear friends.
    
    - Limit grub_malloc() on x86_64 to < 31bit addresses, as some devices seem to
      have a colossally broken storage controller (or UEFI driver) that can't do
      DMA to higher memory addresses, but fails silently.
      Resolves: rhbz#1626844 (possibly really resolving it this time.)
    - Also integrate Hans's attempt to fix the related error from -54, but do it
      the other way around: try the low addresses first and *then* the high one if
      the allocation fails.  This way we'll get low regions by default, and if
      kernel/initramfs don't fit anywhere, it'll try the higher addresses.
      Related: rhbz#1624532
    - Coalesce all the intermediate debugging junk from -54/-55/-56.
    
    Signed-off-by: Peter Jones <pjones@redhat.com>