Add support for new "stat.big-num" boolean option.

This allows a user to set a default for "--no-big-num" for "perf stat"
commands.

--
  $ perf config stat.big-num
  $ perf stat --event cycles /bin/true

   Performance counter stats for '/bin/true':

             778,849      cycles
  [...]
  $ perf config stat.big-num=false
  $ perf config stat.big-num
  stat.big-num=false
  $ perf stat --event cycles /bin/true

   Performance counter stats for '/bin/true':

              769622      cycles
  [...]
--

There is an interaction with "--field-separator" that must be
accommodated, such that specifying "--big-num --field-separator={x}"
still reports an invalid combination of options.

Documentation for perf-config and perf-stat updated.
Signed-off-by: Paul Clarke <pc@us.ibm.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: http://lore.kernel.org/lkml/1589991815-17951-1-git-send-email-pc@us.ibm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>