perf_counter: add sample user-space to Documentation/perf_counter/

Initial version of kerneltop.c and perfstat.c.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>