[seL4] config-independent object type numbers

Right now, object type numbers vary based on the kernel config (eg, CONFIG_VTX) etc. This is a huge source of pain for the Rust userspace libraries which don't integrate with seL4's build system. Is it possible for those enums to explicitly label every potential object type for a given architecture with a fixed number? -- cmr http://octayn.net/ +16038524272