wasm: properly pin the raiseJSException closure

We used to use keepAlive# to pin the raiseJSException closure when
blocking on a JSFFI import thunk, since it can potentially be used by
RTS. But raiseJSException may be used in other places as well (e.g.
the promise.throwTo logic), and it's better to simply unconditionally
pin it in the JSFFI initialization logic.