perf_counter tools: Reuse event_name() in kerneltop

- can handle sw counters now
- the outputs will look slightly different

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