kernel/sched: Clean up tracing hooks

The tracing fixes in commit e8719389 ("subsys: debug: tracing: Fix
thread tracing") were... not a readability win.  The point appears to
have been to put a tracing hook immediately before and after the
assignment to the _current pointer.  So do that in an abstracted
function and clean up _get_next_switch_handle() (which is a subtle and
important function already polluted with some unavoidable preprocessor
testing!)

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>