stack_sentinel: change cooperative check

One of the stack sentinel policies was to check the sentinel
any time a cooperative context switch is done (i.e, _Swap is
called).

This was done by adding a hook to _check_stack_sentinel in
every arch's __swap function.

This way is cleaner as we just have the hook in one inline
function rather than implemented in several different assembly
dialects.

The check upon interrupt is now made unconditionally rather
than checking if we are calling __swap, since the check now
is only called on cooperative _Swap(). The interrupt is always
serviced first.

Issue: ZEP-2244
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>