Make 64-on-32 maybe work on x86_64.

Make 64-on-32 maybe work on x86_64.

This is mostly based on a patch (https://github.com/mjg59/shim/issues/30)
from https://github.com/TBOpen , which refactors our __LP64__
tests to be tests of the header magic instead.  I've simplified things
by using what we've pre-loaded into "context" and making some helper
functions so the conditionals in most of the code say what they do,
instead of how they work.

Note that we're only allowing that from in_protocol's loader - that is,
we'll let 64-bit grub load a 32-bit kernel or 32-bit grub load a 64-bit
kernel, but 32-bit shim isn't loading a 64-bit grub.

Signed-off-by: Peter Jones <pjones@redhat.com>