aco/ngg: Implement NGG GS output.

We store emitted GS vertices in LDS.
Then, at the end of the shader, the emitted vertices are compacted
and each thread loads a single vertex from LDS in order to export
a primitive as needed, and the vertex attributes.

The reason this	is done is because there is an impedance mismatch
between	how API	GS and the NGG HW works. API GS can emit an arbitrary
number of vertices and primites	in each	thread,	but NGG	HW can only
export one vertex per thread.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <!6964>