Add NT_FREEBSD_FEATURE_CTL ELF note to csu

This note will be used to allow binaries to opt out of, or in to,
upcoming vulnerability mitigation and other features. It is not yet
connected but being added now to facilitate testing and ensure
compatibility with existing kernels and tools.

Reviewed by: brooks, jhb, kib, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17438