This fixes the Stack Clash issue rediscovered by Qualys. See
https://www.qualys.com/2017/06/19/stack-clash/stack-clash.txt
for more information on the topic, specifically section III.
We don't have the kernel mitigation available because it is a Grsecurity
feature which we don't support anymore. Other distributions like Gentoo
Hardened and Arch already have `-fstack-check` enabled by default.
See the Gentoo page on Stack Clash for more information on this solution:
https://wiki.gentoo.org/wiki/Hardened/Gentoo_Hardened_and_Stack_Clash
This unfortunately doesn't apply to clang because `-fstack-check` is a
noop there. Note that the GCC implementation also has problems that could
be exploited to circumvent these checks but it is still better than
keeping it disabled.
4150f5e8ba