radv/ci: add a job to test the LLVM backend on navi21

The LLVM backend is not officially supported by the RADV developers, but it has been useful early during bring-up, or later when users are experiencing what looks like a compiler bug. It is thus beneficial to keep it working.

However, maintaining the vkcts expectations for every platform requires more work and machine time than what we would like to commit to. This is why we agreed that we would only keep LLVM tested on the latest family of Radeon GPUs.