contrib: fxc2: Build an aarch64 version of fxc2 if doing the build on aarch64

Bump to a newer version of the fxc2 repo, that support aarch64.

The fxc2 tool in contribs isn't meant to be built for the cross target, but to run on the build host; therefore, pick an aarch64 mingw cross compiler if running on aarch64 linux.