Use the in-tree sys/elf_common.h to build libelftc.

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