radv: Allow building when LLVM isn’t enabled

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
is an issue for debugging, so I added a warning that this configuration
is unsupported.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11319>