Unify notion of runtime "aliveness"

Currently we have 3 different ways to tell in JS current runtime is alive.

- `ABORT` which gets set from the `abort()` function. When this is true the runtime should be considered dead.
- `runtimeExited`, which which is set when the runtime is shut down. This not always available (for example with `MINIMAL_RUNTIME` + `EXIT_RUNTIME=0`
- A pthread work runtime is considered dead if it is not hosting a pthread i.e. `pthread_self()` returns 0.

We should try to unify these.