threads: add per-thread "return" values for 'model-checking/user context' switch

threads: add per-thread "return" values for 'model-checking/user context' switch

The model-checker needs to return a value to the user context when performing
atomic loads, for instance. I will be implementing this by caching the return
value on a per-thread basis. This is because an atomic_load() might result in a
context switch before actually returning the value.

These functions are not yet used.