perf stat: Refactor __run_perf_stat() common code

This extracts common code from the branches of the forks if-then-else.

enable_counters(), which was at the beginning of both branches of the
conditional, is now unconditional; evlist__start_workload() is extracted
to a different if, which enables making the common clocking code
unconditional.

Reviewed-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Adrián Herrera Arcila <adrian.herrera@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@arm.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: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/r/20220729161244.10522-1-adrian.herrera@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>