panfrost: gen_pack: Add the aggregate concept

panfrost: gen_pack: Add the aggregate concept

Panfrost descriptors are big and are usually built from a combination of
sub-descriptors. On top of that, layout of sub-descriptors might vary
depending on the architecture version. Since unions are not really an
option (too complex), here is a thin abstraction layer allowing us to
manipulate aggregates in their packed format. Each aggregate is formed
of one or more sections that are meant to be packed/unpacked/printed
separately. Section overlapping is allowed to facilitate handling of
descriptor variants.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6797>