llvmpipe: add compute shader generation.

llvmpipe: add compute shader generation.

This creates the coroutine execution environment and the
main compute shaders that get executed inside it.

Each compute shader block is executed in it's own coroutine
execution shader, which each "thread" being a coroutine executed
inside it in sequence.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>