Fix a linkage error when cross building for windows with the hardening flags

Summary:
The linker is missing the hardened functions when _FORTIFY_SOURCE > 0.
It used to be supplied by libssp that was removed in D17229, but this implies the use of mingw >= 11 when we still use 10 for most of our CI. Let's keep this linked in for now and improve the comment.

Test Plan: Run the cross windows build.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17248