x86: Properly initialize _interrupt_vectors_allocated[] bitmap

Updates the 'gen_idt' tool to generate a bitmap of (statically) allocated
interrupt vectors that is linked into the final image in a manner similar to
the static IDT. The kernel then uses this bitmap when dynamically connecting
an interrupt vector, thereby preventing the dynamic irq connections
from clobbering the static irq connections.

Change-Id: I0a8f488408dad4912736865179f32f63ff1ca98f
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>