Fix a bug in codegen for non-updatable selector thunks (#8817)

To evaluate most non-updatable thunks, we can jump directly to the
entry code if we know what it is.  But not for a selector thunk: these
might be updated by the garbage collector, so we have to enter the
closure with an indirect jump through its info pointer.
fix comment on allocate() (#8254)
Add hs_thread_done() (#8124)

See documentation for details.