radv: add a LLVM version string workaround for SotTR and ACO

radv: add a LLVM version string workaround for SotTR and ACO

When the LLVM version is too old or missing, SotTR applies shader
workarounds and that reduces performance by 2-5% with ACO.

SotTR workarounds are applied with LLVM 8 and older, so reporting
LLVM 9.0.1 should be fine.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Edmondo Tommasina <edmondo.tommasina@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4984>