This fixes build failures on older releases that lack various
definitions such as EM_AARCH64 (which was unfixed before this).
Revert all of the recent compatibility changes that worked around this
problem.
This uses the same method of using the in-tree header as lib/libelf,
lib/libdwarf and usr.bin/readelf.
Reviewed by: emaste
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D6734
3456e2d15b