radv: enable shaders cache for libraries with GPL

radv: enable shaders cache for libraries with GPL

This was the last missing feature for GPL. The main problem is that
the on-disk shaders cache size will increase a lot because we don't
deduplicate shaders but there is on-going work to improve that.

We also can't use the shaders cache for libraries created with the
RETAIN_LINK_TIME_OPTIMIZATION flag and module identifiers because we
don't know the SPIR-V and thus can't retain NIR shaders for linking.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22264>