hle: kernel: Fix service_threads access to be thread safe V2.

- PR #7699 attempted to fix CreateServiceThread and ReleaseServiceThread to be thread safe, but inadvertently introduced a possible dead-lock.
- With this PR, we use a worker thread to manage the service thread list, allowing it only to be accessed by a single thread, and guaranteeing threads will not destroy themselves.
- Fixes a rare crash in Pokemon Sword/Shield, I've now run this game for ~12 hours non-stop and am quite confident this is a good solution for this issue.