state_tracker, iris support for fp64-lowering with nir

@airlied: Using your st/mesa: add support for lowering fp64/int64 for nir drivers, but with some significant changes.

@mattst88: Possibly of interest to you. Also, in st/mesa: add support for lowering fp64/int64 for nir drivers, is it correct where I only compile the fp64 functions when nir_lower_fp64_full_software is set? This varies a bit from the i965 code, but seemed correct.

@kwg: Is this what you meant with regards to nir_shader_compiler_options?