x86: UV, SGI RTC: add generic system vector, build fix on UP

Make ack_APIC_irq() build on !SMP && !APIC too.

Cc: Dimitri Sivanich <sivanich@sgi.com>
LKML-Reference: <20090304185605.GA24419@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>