readelf: Add -z decompression support

Compatible with GNU readelf, -z decompresses sections displayed by
-x or -p.

ELF Tool Chain ticket #555
https://sourceforge.net/p/elftoolchain/tickets/555/

Submitted by: Tiger Gao <tig@FreeBSDFoundation.org>
Reviewed by: markj
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26909