ssp: appease -Wgnu-statement-expression-from-macro-expansion

It's a stupid warning, but some ports enable it by default and were
already defining _FORTIFY_SOURCE, thus exposing the new macros
immediately. This at least fixes the libfido2 build, perhaps others as
well.

While we're here, fix a fresh build of stand w/ FORTIFY_SOURCE enabled
by not pulling in the ssp headers if _STANDALONE is defined. We do not
have runtime support in libsa as of the time of writing.

Reported by: netchild
Sponsored by: Stormshield
Sponsored by: Klara, Inc.

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