Replace various macros from compartment-macros with compiler builtins.

[davidchisnall]

A lot of things in compartment-macros.h that should be compiler builtins:

• MMIO_CAPABILITY, should be __builtin_cheriot_mmio_capability(type, identifier), lowered to llvm.cheriot.mmio_capability, which is attributed so that optimisers know that it always returns the same value, and then expanded to something equivalent to the current assembly in the back end (ideally with the permissions handled more sensibly).
• CHERIOT_EMIT_STATIC_SEALING_TYPE should be __builtin_cheriot_sealing_type(identifier), generating an LLVM intrinsic that then expanding in the back end to something equivalent to the current assembly.
• DECLARE_STATIC_SEALED_VALUE should be replaced with a [[cheriot::sealed(identifier, identifier)]] attribute on a structure. This should use the attribute that will be introduced in Add type attributes (and wire them through name mangling) for sealed types #5, the only thing allowed on these globals is taking their address. STATIC_SEALED_VALUE then is replaced with simply taking the address of a global declared with the cheriot::sealed attribute.