Add tool to modify ELF binary feature control bits

This will allow feature control bits (e.g. for ASLR, PROT_MAX) to be
inspected or modified.

Some clean-up and additional work is likely still required, but we can
iterate on this in the tree.

Submitted by: Bora Özarslan <borako.ozarslan@gmail.com>
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19290