perf util: Add evsel__taskstate() to parse the task state info instead

perf util: Add evsel__taskstate() to parse the task state info instead

Now that we have the __prinf_flags() parsing routines, we add a new
helper evsel__taskstate() to extract the task state info from the
recorded data.

Signed-off-by: Ze Gao <zegao@tencent.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20240122070859.1394479-5-zegao@tencent.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>