radv: Add the concept of radv shader binaries.

radv: Add the concept of radv shader binaries.

This simplifies a bunch of stuff by
(1) Keeping all the things in a single allocation, making things easier
 for the cache.
(2) creating a shader_variant creation helper.

This is immediately put to use by creating rtld shader binaries. This
is the main reason for the binaries, as we need to do the linking at
upload time, i.e. post caching. We do not enable rtld yet.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>