Draft: nir,agx: Implement transform feedback

Only the last 4 commits of this MR. The rest is !20562 (merged), !20558 (merged), and shader variants work that I haven't made an MR for.

Draft because this is fundamentally broken, even though it passes the dEQPs. Then again, it's just the panfrost implementation......

MR to get the discussion started, I guess. I'd be tempted to land and hide behind =deqp but meh.

Cc @asahilina