MINOR: sched: add a new function is_sched_alive() to report scheduler's health

MINOR: sched: add a new function is_sched_alive() to report scheduler's health

This verifies that the scheduler is still ticking without having to
access the activity[] array nor keeping local copies of the ctxsw
counter. It just tests and sets a flag that is reset after each
return from a ->process() function.