The 8259a PIC has a well known problem of generating flaky
IRQ7 interrupts. The correct solution is to always check
if an IRQ7 interrupt is real or not by probing the PIC's ISR
register. This check is only mandatory if the IRQ7 is actually
being used by the system. More importantly, the handler should
NEVER send and EOI if the interrupt was spurious.
This patch addresses this issue by implementing a fake empty
handler for this IRQ and, as stated, NOT sending the EOI.
3b01e04379