perf tools: Default to cpu// for events v5

perf tools: Default to cpu// for events v5

When an event fails to parse and it's not in a new style format,
try to parse it again as a cpu event.

This allows to use sysfs exported events directly without //, so you can use

perf record -e mem-loads ...

instead of

perf record -e cpu/mem-loads/

Signed-off-by: Andi Kleen <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>