kernel: update z_sched_thread_usage()

This commit does two things to the z_sched_thread_usage(). First,
it updates the API so that it accepts a pointer to the runtime
stats instead of simply returning the usage cycles. This gives it
the flexibility to retrieve additional statistics in the future.

Second, the runtime stats are only updated if the specified thread
is the current thread running on the current core.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>