Provide stack smashing protection for mingw-clang builds

Currently with our mingw-w64/gcc builds we provide among other things stack smashing protection (SSP). We want to do the same for mingw-w64/clang builds.

https://bugzilla.mozilla.org/show_bug.cgi?id=1511073 is the upstream bug. https://bugzilla.mozilla.org/show_bug.cgi?id=1503589#c19 has a good summary to get us started (pointing e.g. to https://github.com/mstorsjo/llvm-mingw/blob/master/build-libssp.sh)