ac,radv,radeonsi: remove LLVM 7 support

Now that LLVM 9 will be released soon, we will only support
LLVM 8, 9 and master (10).

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
14 files changed