Add afterbuild target to bsd.prog.mk.

Afterbuild target allows to perform operations on fully built binary.
This is needed to allow for ELF feature flags modification during
world build.

Submitted by: Dawid Gorecki <dgr@semihalf.com>
Reviewed by: imp
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D29551