OFBlockTests: Also disable global blocks on Win64

While these work on the officially by ObjFW supported MSYS2/CLANG64
environment, they don't work on the similar environment provided by
Fedora in the ucrt64-gcc package when using
OBJC='clang -target x86_64-w64-mingw32ucrt', which is not officially
supported by ObjFW, but useful for testing purposes.