spirv: Convert the supported_extensions struct to spirv_options

spirv: Convert the supported_extensions struct to spirv_options

This is a bit more general and lets us pass additional options into the
spirv_to_nir pass beyond what capabilities we support.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>