How difficult is it to enumerate the data-structure and component boundaries within the compiler where target-architecture annotation would need to be added? Presumably proc-macros would be just a virtual target-architecture, though probably a distinguished member of the target-architecture enumeration, along with the compiler's host target-architecture.
This same effort could also lay the groundwork for the modular ABI proposal that is in a concurrent thread, as in some sense those different ABIs can be considered to be alternate target-architectures.