perf ftrace: Add support for trace option tracing_thresh

This adds an option '--graph-opts thresh' to setup trace duration
threshold for funcgraph tracer.

  $ sudo ./perf ftrace -G '*' --graph-opts thresh=100
   3) ! 184.060 us  |    } /* schedule */
   3) ! 185.600 us  |  } /* exit_to_usermode_loop */
   2) ! 225.989 us  |    } /* schedule_idle */
   2) # 4140.051 us |  } /* do_idle */

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: http://lore.kernel.org/lkml/20200808023141.14227-14-changbin.du@gmail.com


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