Fix fxc2 usage for Firefox 78 ESR

The Firefox build system is looking for wine64 now first (see: https://bugzilla.mozilla.org/show_bug.cgi?id=1619504) which seems, somewhat surprisingly, break our build.

We should probably simplify our fxc2 setup as we don't need to build and run 64 AND 32bit versions. It's kind of a host tool (even though it needs to be cross-compiled). Thus, just having a 64bit version for the shader file generation is fine.