perf tools: Allow sorting by symbol size

Add new sort key 'symbol_size' to allow user to sort by symbol size, or
(more usefully) display the symbol size using --fields=...,symbol_size.

Committer note:

Testing it together with the recently added -q, to remove the headers,
and using the '+' sign with -s, to add the symbol_size sort order to
the default, which is '-s/--sort comm,dso,symbol':

  # perf report -q -s +symbol_size | head -10
  10.39%  swapper       [kernel.vmlinux] [k] intel_idle               270
   3.45%  swapper       [kernel.vmlinux] [k] update_blocked_averages 1546
   2.61%  swapper       [kernel.vmlinux] [k] update_load_avg         1292
   2.36%  swapper       [kernel.vmlinux] [k] update_cfs_shares        240
   1.83%  swapper       [kernel.vmlinux] [k] __hrtimer_run_queues     606
   1.74%  swapper       [kernel.vmlinux] [k] update_cfs_rq_load_avg. 1187
   1.66%  swapper       [kernel.vmlinux] [k] apic_timer_interrupt     152
   1.60%  CPU 0/KVM     [kvm]            [k] kvm_set_msr_common      3046
   1.60%  gnome-shell   libglib-2.0.so.0 [.] g_slist_find              37
   1.46%  gnome-termina libglib-2.0.so.0 [.] g_hash_table_lookup      370
  #

Signed-off-by: Charles Baylis <charles.baylis@linaro.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Maxim Kuvyrkov <maxim.kuvyrkov@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1487943176-13840-1-git-send-email-charles.baylis@linaro.org
[ Use symbol__size(), remove needless %lld + (long long) casting ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>