Integrate fxc2 into our build setup for Windows Tor Browser builds

Starting with https://bugzilla.mozilla.org/show_bug.cgi?id=1365859 Mozilla is using a closed-source tool, called fxc, during the build to autocreate D3D11 shaders during the build.

We don't want to do that. So, tjr set down and wrote a tool, called fxc2 (https://github.com/tomrittervg/fxc2), that we can compile and then use during the build via wine(!) to do the autogeneration for us. See: https://bugzilla.mozilla.org/show_bug.cgi?id=1370007 for details.

We need to integrate that setup into our Windows build process for Tor Browser 8.