Windows-only toolchain updates.

Also, bug 40832: Unify mingw-w64-clang 32+64 bits

The clang binaries used in the mingw-w64-clang artifacts were exactly
the same, because Clang allows you to set the target at runtime with
CLI options, contrarily to GCC, that allows you to set them at build
time.

Also, i686 and x86_64 parts that differ have different paths, so they
can live in the same mingw-w64 installation.
This allows us to save both time, and space on the out directory, since
we can safely use only one archive for both platforms.

Also, partially bug 40652: drop dependency on libssp.

Libssp is not necessary anymore for _FORTIFY_SOURCE.
So, we match llvm-mingw, and replace it with empty archives.
In the future, we should stop creating these archives, too, when the
compiler users stop including it during linking.