Kernel: Allow to install a real IRQ handler on a spurious one

IRQ 7 and 15 on the PIC architecture are used for spurious interrupts.
IRQ 7 could also be used for LPT connection, and IRQ 15 can be used for
the secondary IDE channel. Therefore, we need to allow to install a
real IRQ handler and check if a real IRQ was asserted. If so, we handle
them in the usual way.

A note on this fix - unregistering or registering a new IRQ handler
after we already registered one in the spurious interrupt handler is
not supported yet.

Kernel/Interrupts/SpuriousInterruptHandler.h

@@ -58,6 +58,7 @@ private:
     void disable_interrupt_vector();
     explicit SpuriousInterruptHandler(u8 interrupt_number);
     bool m_enabled;
+    bool m_real_irq { false };
     RefPtr<IRQController> m_responsible_irq_controller;
     OwnPtr<GenericInterruptHandler> m_real_handler;
 };