Revert "sched: remove redundant update_runtime notifier"

This reverts commit 4373a4647bfa8b33ddee3e9e82d6a7ae8809fa4e.
Revert "sched: reinitialize rq->next_balance when a CPU is hot-added"

This reverts commit 2d8b1263b3a5b1ed922d3a0ef2d32b68c8855725.