r600/sfn: Add experimental support for rustcl
The MR adds some support for global_load and global_store, it only can deal with up to vec4 float or vec2 double values, and images are not supported at all - I think r600 on Clover also doesn't support these. AFAICS the problem there doesn't lie in the shader, but in the image state set-up.
Enabling the backend still needs export RUSTICL_ENABLE=r600