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>

arch/x86_64/include/kernel_arch_func.h

@@ -42,6 +42,15 @@ static inline void z_arch_irq_unlock(unsigned int key)
 	}
 }
 
+/**
+ * Returns true if interrupts were unlocked prior to the
+ * z_arch_irq_lock() call that produced the key argument.
+ */
+static inline bool z_arch_irq_unlocked(unsigned int key)
+{
+	return (key & 0x200) != 0;
+}
+
 static inline void arch_nop(void)
 {
 	__asm__ volatile("nop");