perf stat display: Make %f precision consistent

Commit bc22de9bcdb22491 ("perf stat: Display time in precision based on
std deviation") added multirun workload elapsed time. There was an
effort to make the precision in the output most useful for the user,
however, when gathering over runs it means the formatting varies. This
change just makes the output format fixed.

Before:
```
$ while :; do perf stat --null --repeat 3 sleep 0.1 2>&1 | grep elapsed; done
0.101140 +- 0.000149 seconds time elapsed ( +- 0.15% )
0.1011396 +- 0.0000218 seconds time elapsed ( +- 0.02% )
0.101331 +- 0.000124 seconds time elapsed ( +- 0.12% )
^C
$ while :; do perf stat --null --repeat 3 sleep 1 2>&1 | grep elapsed; done
1.001317 +- 0.000146 seconds time elapsed ( +- 0.01% )
1.001377 +- 0.000172 seconds time elapsed ( +- 0.02% )
1.00253 +- 0.00131 seconds time elapsed ( +- 0.13% )
```

After:
```
$ while :; do perf stat --null --repeat 3 sleep 0.1 2>&1 | grep elapsed; done
0.101406408 +- 0.000064778 seconds time elapsed ( +- 0.06% )
0.101367315 +- 0.000027253 seconds time elapsed ( +- 0.03% )
0.101434164 +- 0.000084750 seconds time elapsed ( +- 0.08% )
^C
$ while :; do perf stat --null --repeat 3 sleep 1 2>&1 | grep elapsed; done
1.001525467 +- 0.000051703 seconds time elapsed ( +- 0.01% )
1.001375093 +- 0.000116200 seconds time elapsed ( +- 0.01% )
1.001141025 +- 0.000046361 seconds time elapsed ( +- 0.00% )
```

Closes: https://lore.kernel.org/lkml/aTQRgAOpKyI53TEq@gmail.com/
Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Ingo Molnar <mingo@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Chun-Tse Shao <ctshao@google.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>