kernel/sched: fix thread selection when ABORTING + PENDING

In commit d537267f, the check on thread abortion was moved from next_up
to z_get_next_switch_handle. However, next_up is also called from
z_swap_next_thread, so the check on thread abortion is now missing there.
This sometimes caused the thread to be stuck in ABORTING + PENDING state
during the test_smp_switch_torture in test/kernel/smp

To avoid such cases in the future, it is worth leaving the check in next_up

Signed-off-by: Vadim Shakirov <vadim.shakirov@syntacore.com>