Move ELF feature note tool to usr.bin/elfctl

elfctl is a tool for modifying the NT_FREEBSD_FEATURE_CTL ELF note,
which contains a set of flags for enabling or disabling vulnerability
mitigations and other features.

Reviewed by: csjp, kib
MFC after: 2 weeks
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23910