perf stat: Use field separator in the metric header

perf stat: Use field separator in the metric header

It didn't use the passed field separator (using -x option) when it
prints the metric headers and always put "," between the fields.

Before:
  $ sudo ./perf stat -a -x : --per-core -M tma_core_bound --metric-only true
  core,cpus,%  tma_core_bound:     <<<--- here: "core,cpus," but ":" expected
  S0-D0-C0:2:10.5:
  S0-D0-C1:2:14.8:
  S0-D0-C2:2:9.9:
  S0-D0-C3:2:13.2:

After:
  $ sudo ./perf stat -a -x : --per-core -M tma_core_bound --metric-only true
  core:cpus:%  tma_core_bound:
  S0-D0-C0:2:10.5:
  S0-D0-C1:2:15.0:
  S0-D0-C2:2:16.5:
  S0-D0-C3:2:12.5:

Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: https://lore.kernel.org/r/20240628000604.1296808-2-namhyung@kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>