x86, perf: Clean up perf_event cpu code

The CPU support for perf events on x86 was implemented via included C files
with #ifdefs.  Clean this up by creating a new header file and compiling
the vendor-specific files as needed.

Signed-off-by: Kevin Winchester <kjwinchester@gmail.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1314747665-2090-1-git-send-email-kjwinchester@gmail.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>