perf diff: Add weighted diff computation way to compare hist entries

Adding 'wdiff' as new computation way to compare hist entries.

If specified the 'Weighted diff' column is displayed with value 'd'
computed as:

   d = B->period * WEIGHT-A - A->period * WEIGHT-B

  - A/B being matching hist entry from first/second file specified
    (or perf.data/perf.data.old) respectively.
  - period being the hist entry period value
  - WEIGHT-A/WEIGHT-B being user suplied weights in the the '-c' option
    behind ':' separator like '-c wdiff:1,2'.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1349448287-18919-5-git-send-email-jolsa@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>