pan/va: Add Valhall assembler

pan/va: Add Valhall assembler

This Python script acts as a standalone assembler. It takes Valhall
instructions with Mesa-flavour syntax, parses them, errors out if there
are syntax errors, and writes out an assembled binary if there are not.
It also is available as a programmatic interface for automated testing.

While this attempts to handle syntax errors, it does not check
semantics. It will happily compile programs that fault, provided each
instruction locally 'looks' plausible.

The code itself is quite small, despite supporting most of the known
ISA, because the syntax is regular and the heavylifting is done by
ISA.xml and valhall.py.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12025>