x86: Update generation of static IDT

Adds two new fields to the ISR_LIST structure (irq and priority) to allow
the decoupling of the vector ID and priority from the IRQ number at some
future time.

As a result of the addition of these two new fields, the gen_idt tool is
modified to both process these new fields as well as validate them.

Change-Id: I343dac68d99c78168a25b19784140f85d5db7578
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>