arch/*: Add z_arch_irq_unlocked() predicate and test

It's useful to be able to inspect the key returned from
z_arch_irq_unlock() to see if interrupts were enabled at the point
where z_arch_irq_lock() was called.  Architectures tend to represent
this is a simple way that doesn't require platform assembly to
inspect.

Adds a simple test to kernel/common that validates this predicate with
a nested lock.

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