Ideally, all of the changes necessary in OpenSSL, libffi, and Tcl/Tk will happen upstream and we'll just update to new versions of them.  We have PR 3806 in progress to extract libffi from our repo and treat it just as any other external dependency on Windows, but neither Steve nor I have had a chance to work on it in quite a long time.  That will probably be a prerequisite for properly supporting ARM64 Windows.