radeonsi: set shader calling conventions

radeonsi: set shader calling conventions

Note that old mesa + new LLVM or new mesa + old LLVM breaks
with this change and the corresponding LLVM change (D18559).

For LLVM version <= 3.8 we use the old method, but we can't detect
people using a post 3.8 svn version that is still too old.

Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>