radv: Allow building when LLVM isn’t enabled
Now that ACO is considered feature-complete, it can be nice to avoid the huge LLVM dependency when one only wants a Vulkan driver.
This patch allows radv to be built without LLVM. The two features which
get disabled are RADV_DEBUG=llvm
and shader disassembly. The latter
might be an issue for debugging, if that’s needed the user should be
recommended to rebuild Mesa with LLVM support.