Two fixes for the x86 interrupt code:

  - Unbreak the magic 'search the timer interrupt' logic in IO/APIC code
    which got wreckaged when the core interrupt code made the state
    tracking logic stricter. That caused the interrupt line to stay masked
    after switching from IO/APIC to PIC delivery mode, which obviously
    prevents interrupts from being delivered.

  - Make run_on_irqstack_code() typesafe. The function argument is a void
    pointer which is then casted to 'void (*fun)(void *). This breaks
    Control Flow Integrity checking in clang. Use proper helper functions
    for the three variants reuqired.
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