x86: use _IRQ_TO_INTERRUPT_VECTOR() with ioapic_irq_set()

ioapic_irq_set() needs a vector number: all calls to it now use the
_IRQ_TO_INTERRUPT_VECTOR() macro as expected.

The previous change by which interrupt priorities are now honoured
correctly when connecting ISRs to IRQs statically also changed the way
the interrupt vectors are assigned.

Instead of computing them like this:

    interrupt vector ID = IRQ + INT_VEC_IRQ0

They are now determined by a macro:

    interrupt vector ID = _IRQ_TO_INTERRUPT_VECTOR(irq)

Change-Id: Icc4576ac9bc6891c8662bcc17a543333eb8745e0
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>