x86/irq: Add and use NR_EXTERNAL_VECTORS and NR_SYSTEM_VECTORS

Add defines for the number of external vectors and number of system
vectors instead of requiring the use of (FIRST_SYSTEM_VECTOR -
FIRST_EXTERNAL_VECTOR) and (NR_VECTORS - FIRST_SYSTEM_VECTOR)
respectively. Clean up the usage sites.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20210519212154.511983-3-hpa@zytor.com