ac/llvm: don't set unsupported xnack options to fix LLVM crashes on gfx6-8
LLVM prints an error if xnack is unsupported and it uses a global stream object that is not thread-safe. Since Mesa uses multiple threads to compile shaders, there is a small chance that it will crash. Just don't set any xnack options to use LLVM defaults. Closes: mesa/mesa#4439 Cc: 20.3 21.0 <mesa-stable@lists.freedesktop.org> Reviewed-by:Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com> Part-of: <mesa/mesa!9795>