kernel: Add custom scheduler yield routines

Adds customized yield implementations based upon the selected
scheduler (dumb, multiq or scalable). Although each follows the
same broad outline, some of them allow for additional tweaking
to extract maximal performance. For example, the multiq variant
improves the performance of k_yield() by about 20%.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>