kernel/swap: Move arch_cohere_stacks() back under the lock

Commit 6b84ab38 ("kernel/sched: Adjust locking in z_swap()") moved
the call to arch_cohere_stacks() out of the scheduler lock while doing
some reorgnizing.  On further reflection, this is incorrect.  When
done outside the lock, the two arch_cohere_stacks() calls will race
against each other.

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