Factor out handling of architecture-dependent snapshot state #1694

Currently, there are no architecture-dependent hooks that can add things to snapshot/restore state. Because of this, as of #991, all of the ostensibly architecture-independent code on the snapshot save/restore path ends up with a bunch of #[cfg(target_arch = "x86_64")] state threaded through it. It would be nice to factor that out into something that makes it easier to add architecture-dependent state.

(This issue extracted from a comment on #991: https://github.com/hyperlight-dev/hyperlight/pull/991/changes#r3625046429)