kernel/sched: Clean up tracing hooks
kernel/sched: Clean up tracing hooks

The tracing fixes in commit e87193896a83 ("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 <c70f9a6bf6ee0cd69c8af4ce5e6b131945769315@intel.com>

GiulianoFranchetto/zephyr,nashif/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,ldts/zephyr,Vudentz/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr