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.