perf_counter, x86: make interrupt handler model specific

This separates the perfcounter interrupt handler for AMD and Intel
cpus. The AMD interrupt handler implementation is a follow-on patch.

[ Impact: refactor and clean up code ]

Signed-off-by: Robert Richter <robert.richter@amd.com>
Cc: Paul Mackerras <paulus@samba.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1241002046-8832-9-git-send-email-robert.richter@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>