perf tests stat+csv_output: Switch CSV separator to @

perf tests stat+csv_output: Switch CSV separator to @

Commas may appear in events like:

  cpu/INT_MISC.RECOVERY_CYCLES,cmask=1,edge/

which causes the commachecker to see more fields than expected. Use @ as
the CSV separator to avoid this.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Claire Jensen <cjense@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Link: https://lore.kernel.org/r/20230223071818.329671-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>