kernel: Minor optimization to kernel event logger timestamping

Rewrites the timestamping logic to always generate timestamps
via a function pointer that is initialized to sys_cycle_get_32(),
but can be changed to point to a user-supplied function. This
eliminates the need for an if/then/else construct in every place
that a timestamp is generated.

Change-Id: Id11f8c41b193a93cece16565978a525056010f0e
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>