[CHERIoT]: Add cheriot_sealing_type clang built-in

[xdoardo]

This PR introduces a new __builtin_cheriot_sealing_type built-in command in clang, that should replace the CHERIOT_EMIT_STATIC_SEALING_TYPE macro, as referenced in #6. Its syntax resembles that of a call to a C++ template function:

__builtin_cheriot_sealing_type<MySealingType>()

Of course, it can be used in plain C as well, with the same syntax.

The side effects of this call is that two new entries,

__import.sealing_type.<current_compartment>.MySealingType

and

__export.sealing_type.<current_compartment>.MySealingType

are generated, respectively in the .compartment_imports and .compartment_exports sections. The value of the import is a sequence of two words, the address of the export and 0; the value of the export reproduces the value generated in the original macro.

[davidchisnall]

I am a bit confused by this. This builtin needs to take an identifier not a type. The same type can be used for two kinds of sealed value.