intel/compiler: add function support
For proper rusticl support we need function calling support in the backends via the nir driver_functions option.
This is just a tracker issue to make sure it's on the project's radar.
I can find a little bit of info in: https://github.com/intel/intel-graphics-compiler/blob/master/documentation/visa/3_execution_model.md
but someone might know if there's an Intel defined calling convention that we could be compatible with in Mesa, not sure how much the need to be compatible is, but it might be good to just copy it.