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>

include/arch/nios2/arch.h

@@ -124,6 +124,15 @@ static ALWAYS_INLINE void z_arch_irq_unlock(unsigned int key)
 #endif
 }
 
+/**
+ * Returns true if interrupts were unlocked prior to the
+ * z_arch_irq_lock() call that produced the key argument.
+ */
+static ALWAYS_INLINE bool z_arch_irq_unlocked(unsigned int key)
+{
+	return key & 1;
+}
+
 void z_arch_irq_enable(unsigned int irq);
 void z_arch_irq_disable(unsigned int irq);