kernel/sched: Predicate SWAP_NONATOMIC workaround properly

This is a refactoring of the fix in commit 6c95dafd to limit its
application to affected platforms now that the root cause is
understood.

Note that the bug that fix was addressing was rare and seen only on
after multi-hour sessions on Michael Scott's test rig.  So if
something regresses, this is where to look!

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