sched: Call sched_exit() when a task is exiting

The scheduler needs to do some book-keeping when a task exits.
Call the scheduler hook sched_exit() that takes care of all of
that necessary book-keeping.

Change-Id: I551aead70248b06f9d918e6d032075d6ecaa7fed
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>