sched_ext: Use runnable_at for the default core-sched task ordering

The default core-sched ordering runs the longest waiting task first by
comparing p->scx.core_sched_at stamps. The stamp is maintained under two
rules. touch_core_sched() stamps when a task starts waiting for a CPU and
when its slice runs out. If the scheduler implements
ops.core_sched_before(), touch_core_sched_dispatch() re-stamps on every
dispatch.

A comparison can see one stamp taken under each rule, which isn't a
meaningful ordering. The dispatch rule also buys little - it only aligns
bypass-mode comparisons with the local DSQ order. Multiple schedulers make
the mixed comparisons more common.

Wait time is what p->scx.runnable_at already tracks for the stall watchdog.
Delete core_sched_at with both touch functions and compare runnable_at in
the scx_prio_less() fallback.

runnable_at is refreshed only on enqueue and goes stale while a task keeps
occupying its CPU. Instead of re-stamping, order a running task after every
waiting task as it is the most recently serviced.

Signed-off-by: Tejun Heo <tj@kernel.org>

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