Fixes JuliaLang#49841.

Follow-up to JuliaLang#49774.

This PR makes two changes:
1. It locks `atexit_hooks` while iterating the hooks during execution of `_atexit()` at shutdown.
    - This prevents any data races if another Task is registering a new atexit hook while the hooks are being evaluated.
2. It defines semantics for what happens if another Task attempts to register another atexit hook _after all the hooks have finished_, and we've proceeded on to the rest of shutdown.
    - Previously, those atexit hooks would be _ignored,_ which violates the user's expectations and violates the "atexit" contract.
    - Now, the attempt to register the atexit hook will **throw an exception,** which ensures that we never break our promise, since the user was never able to register the atexit hook at all.
    - This does mean that users will need to handle the thrown exception and likely do now whatever tear down they were hoping to delay until exit.