perf stat aggregation: Add separate thread member

A separate field isn't strictly required. The core field could be
re-used for thread IDs as a single field was used previously.

But separating them will avoid confusion and catch potential errors
where core IDs are read as thread IDs and vice versa.

Also remove the placeholder id field which is now no longer used.

Signed-off-by: James Clark <james.clark@arm.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: John Garry <john.garry@huawei.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Link: https://lore.kernel.org/r/20201126141328.6509-13-james.clark@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>