riot-rs-runqueue: support multiple currently running threads

[elenaf9]

riot-rs-runqueue was designed under the assumption that there is only a single "head" of the runqueue (see get_next, advance, etc.).

In case of multicore, there may be more than one currently running thread.
The two (simplest) alternatives for supporting this are:

1. Global Scheduling: Use a global runqueue for all threads and cores
2. Partitioned Scheduling: Use a separate runqueue per core

After some research and looking into other embedded OSes, I would propose we go for 1), because:

• For now, we only target platforms with small number of symmetric cores (e.g. the RP2040)
• A global runqueue makes best use of the available capacity
• Thread allocation in case of partitioned scheduling can become quite complex

The runqueue therefore needs to be refactored for global scheduling, so that get_next returns a different thread for each core.

[elenaf9]

Closing because I decided to for an alternative implementation of the multicore scheduler, where the runqueue itself doesn't need to support multicore. Instead, the scheduler simply removes the current thread from the runqueue so that the head of the runqueue doesn't get picked twice.