irq: size _irq_to_interrupt_vector_table

Most systems have far less than 256 IRQ lines available, so
save some bytes in ROM by making this a config option.

On systems with MVIC, omit the table entirely as the mapping
is fixed.

The build cmd_gen_idt is slightly easier to read and will fail
immediately if any of the commands in the sequence error out.

Change-Id: I411f114557591e5cd96b618e6f79f97e8bedadf0
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>