llvmpipe/cs: add task/mesh shader support to compute shader builder.

llvmpipe/cs: add task/mesh shader support to compute shader builder.

This allows generating task and mesh variants of compute shaders.

It adds:
- vertex and primitive outputs support - aos writing.
- payload support
- mesh iface for the output and count callbacks.
- draw_id
- multiple iteration support to the exec fn to allow launches
in multiple passes to reduce memory usage

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23066>