Apply -fstack-clash-protection where possible
It seems we can pick up a newer compiler-based hardening flag:
-fstack-clash-protection. It's supposed to be available with clang 11.0.1, see: https://bugzilla.mozilla.org/show_bug.cgi?id=1588710.
It seems this is mainly for Linux and Android. We should go over GCC-using projects, too, as this flag is supposed to be available there as well.