Add elfctl to toolchain.

Add elfctl program to toolchain to allow modifying ELF feature flags
during system build.

Submitted by: Dawid Gorecki <dgr@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D29552

(cherry picked from commit c6081dea597a475e4bbcc8588ece03ae78b58978)