[Task] RSX: Support more interpolation qualifiers

[kd-11]

More of a note-to-self. Add support for flat attribute interpolation. We have custom interpolation code already, so this shouldn't be too bad now.
There is a decision to be made though, it can be fully integrated at the cost of forcing a new shader cache version, or we can plop it in at the cost of larger shaders. I'll hold off on this until the next shader overhaul.