usr.bin/elfctl: Allow for cross-endian operations.

Detect if host endian is different than target endian and swap
byte order of ELF note fields instead of failing.

Submitted by: Dawid Gorecki <dgr@semihalf.com>
Reviewed by: imp
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D29550