perf stat: Print stalled cycles percentage

Print:

           611,527 cycles
           400,553 instructions             # (  0.71 instructions per cycle )
            77,809 stalled-cycles           # ( 12.71% of all cycles )

        0.000610987  seconds time elapsed

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Link: http://lkml.kernel.org/n/tip-fd6x8r1cpyb6zhlrc4ix8m45@git.kernel.org