sched: extend sched_task_load tracepoint to indicate prefer_idle

Prefer idle determines whether the scheduler prefers an idle CPU
over a busy CPU or not to wake up a task on. Knowing the correct
value of this tunable is essential in understanding placement
decisions made in select_best_cpu().

Change-Id: I955d7577061abccb65d01f560e1911d9db70298a
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>