Factor out handling of architecture-dependent snapshot state

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)

Start with the snapshot save/restore path discussed in issue #1694 and the extracted context from pull request #991. Trace where #[cfg(target_arch = "x86_64")] state is threaded through architecture-independent code, then identify the hooks needed for architecture-dependent snapshot state. Done means the existing behavior is preserved while future architecture-specific state can be added without repeating that threading.

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