perf report: Add 'tgid' sort key

Sometimes we need to analyze the data in process level but current sort
keys only work on thread level.  Let's add 'tgid' sort key for that as
'pid' is already taken for thread.

This will look mostly the same, but it only uses tgid instead of tid.
Here's an example of a process with two threads (thloop).

  $ perf record -- perf test -w thloop

  $ perf report --stdio -s tgid,pid -H
  ...
  #
  #    Overhead  Tgid:Command / Pid:Command
  # ...........  ..........................
  #
     100.00%     2018407:perf
         50.34%     2018407:perf
         49.66%     2018409:perf

Suggested-by: Stephane Eranian <eranian@google.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250509210421.197245-1-namhyung@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>