Add stack clash protection to the WITH_SSP flag

Some background info availabe in:
https://best.openssf.org/Compiler-Hardening-Guides/Compiler-Options-Hardening-Guide-for-C-and-C++.html
https://developers.redhat.com/blog/2020/05/22/stack-clash-mitigation-in-gcc-part-3
https://blog.llvm.org/posts/2021-01-05-stack-clash-protection/

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D48651