Loom: Simplify generate_cont_thaw by passing thaw_kind directly

I was staring at this code for x86_32 port, and it seems `exception` implies `return_barrier`. This mostly irks me because we are passing `2` to `Continuation::prepare_thaw` and implicitly coercing it to `bool` during the call. We can instead call with `1` when `return_barrier`.

Further cleanup makes the change more obvious; we can pass `thaw_kind` directly to `generate_cont_thaw`, and thus simplify the code.