Fix the GCC build after _FORTIFY_SOURCE import

We haven't exposed gets(3) in a long time, rip out __gets_chk before
it's too late and something builds a gets(3) user with it enabled.

Import _FORTIFY_SOURCE implementation from NetBSD

This is a mostly-unmodified copy of the various *_chk implementations
and headers from NetBSD, without yet modifying system headers to start
actually including them. A future commit will also apply the needed
bits to fix ssp/unistd.h.

Reviewed by: imp, pauamma_gundo.com (both previous versions), kib
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D32306