perf data convert: Prefer sampled CPU when exporting JSON

When CPU has been explicitly sampled (via --sample-cpu), prefer this
sampled value over the thread CPU value when exporting to JSON.

Signed-off-by: Shawn M. Chapla <schapla@codeweavers.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.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>
Link: https://lore.kernel.org/r/20220526201506.2028281-1-schapla@codeweavers.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>