[PATCH] x86_64 irq: Add constants for the reserved IRQ vectors.

For the ISA irqs we reserve 16 vectors.  This patch adds constants for
those vectors and modifies the code to use them.  Making the code a
little clearer and making it possible to move these vectors in the future.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>