panfrost: Adopt gen_pack_header.py via v3d

panfrost: Adopt gen_pack_header.py via v3d

We fork gen_pack_header.py from v3d, which in turn forks GenXML from
Intel. Selected changes for Mali:

* Custom prefix/vendoring
* Cull supported types
* Drop <register> and <packet> support (no cmdstream)
* Pack 32-bit words instead of 8-bit bytes
* Inline packing helpers
* Introduce exact attribute
* Introduce general minus(..) and shr(..) modifiers
* Introduce word:bit syntax
* Introduce prettyprinters

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6195>