perf stat: Improve readability of shadow stats

perf stat: Improve readability of shadow stats

This adds function convert_unit_double() and selects appropriate
unit for shadow stats between K/M/G.

  $ sudo perf stat -a -- sleep 1

Before: Unit 'M' is selected even the number is very small.

 Performance counter stats for 'system wide':

          4,003.06 msec cpu-clock                 #    3.998 CPUs utilized
            16,179      context-switches          #    0.004 M/sec
               161      cpu-migrations            #    0.040 K/sec
             4,699      page-faults               #    0.001 M/sec
     6,135,801,925      cycles                    #    1.533 GHz                      (83.21%)
     5,783,308,491      stalled-cycles-frontend   #   94.26% frontend cycles idle     (83.21%)
     4,543,694,050      stalled-cycles-backend    #   74.05% backend cycles idle      (66.49%)
     4,720,130,587      instructions              #    0.77  insn per cycle
                                                  #    1.23  stalled cycles per insn  (83.28%)
       753,848,078      branches                  #  188.318 M/sec                    (83.61%)
        37,457,747      branch-misses             #    4.97% of all branches          (83.48%)

       1.001283725 seconds time elapsed

After:

$ sudo perf stat -a -- sleep 2

 Performance counter stats for 'system wide':

          8,005.52 msec cpu-clock                 #    3.999 CPUs utilized
            10,715      context-switches          #    1.338 K/sec
               785      cpu-migrations            #   98.057 /sec
               102      page-faults               #   12.741 /sec
     1,948,202,279      cycles                    #    0.243 GHz
     2,816,470,932      stalled-cycles-frontend   #  144.57% frontend cycles idle
     2,661,172,207      stalled-cycles-backend    #  136.60% backend cycles idle
       464,172,105      instructions              #    0.24  insn per cycle
                                                  #    6.07  stalled cycles per insn
        91,567,662      branches                  #   11.438 M/sec
         7,756,054      branch-misses             #    8.47% of all branches

       2.002040043 seconds time elapsed

v2:
  o do not change 'sec' to 'cpu-sec'.
  o use convert_unit_double to implement convert_unit.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20210315143047.3867-1-changbin.du@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>