spirv: Add explicit pointer types

spirv: Add explicit pointer types

Instead of baking in uvec2 for UBO and SSBO pointers and uint for push
constant and shared memory pointers, make it configurable.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>