perf stat: Move shadow stat counters into separate object

Separating shadow counters code into separate object as a cleanup, but
mainly for upcomming changes, so could use it from script command
context.

Signed-off-by: Jiri Olsa <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>