tu: Rework passing shared consts

tu: Rework passing shared consts

The way this works now is awkward to map to shader objects. We don't
have the pipeline layout when "linking" shaders at draw time, so we have
to piece it together from the shaders. Store the information we need in
the shaders and piece it together.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25276>