Only define `libssp` if our CSL intsall actually has it (#58175)
Some CSL installs (such as those from `USE_BINARYBUILDER=0` builds) do
not have a `libssp`, because their GCC was built without stack smash
protection, or it was statically linked. This allows our CSL_jll to
adapt to that, avoiding issues like those reported in [0].
[0]: https://github.com/JuliaLang/julia/issues/58173
This should fix #58173
