spirv: Add an option for making FragCoord a sysval

spirv: Add an option for making FragCoord a sysval

On AMD, FragCoord should be a sysval because it is handled separately
from all the other inputs. We were already doing this in radeonsi, but
we weren't doing it with radv. It'll be much more annoying to handle
VARYING_SLOT_POS in fragment shaders when we let NIR lower FS inputs for
us, so here we add an option so that radv can get it as a system value.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>