perf report: Implement --sort cpu

In a shared multi-core environment, users want to analyze why their
program was slow. In particular, if the code ran slower only on certain
CPUs due to interference from other programs or kernel threads, the user
should be able to notice that.

Sample usage:

perf record -f -a -- sleep 3
perf report --sort cpu,comm

Workload:

program is running on 16 CPUs
Experiencing interference from an antagonist only on 4 CPUs.

  Samples: 106218177676 cycles

  Overhead  CPU          Command
  ........  ...  ...............

     6.25%  2            program
     6.24%  6            program
     6.24%  11           program
     6.24%  5            program
     6.24%  9            program
     6.24%  10           program
     6.23%  15           program
     6.23%  7            program
     6.23%  3            program
     6.23%  14           program
     6.22%  1            program
     6.20%  13           program
     3.17%  12           program
     3.15%  8            program
     3.14%  0            program
     3.13%  4            program
     3.11%  4         antagonist
     3.11%  0         antagonist
     3.10%  8         antagonist
     3.07%  12        antagonist

Cc: David S. Miller <davem@davemloft.net>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <20100505181612.GA5091@sharma-home.net>
Signed-off-by: Arun Sharma <aruns@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>