perf ftrace: Add option '-F/--funcs' to list available functions

perf ftrace: Add option '-F/--funcs' to list available functions

This adds an option '-F/--funcs' to list all available functions to
trace, which is read from tracing file 'available_filter_functions'.

  $ sudo ./perf ftrace -F | head
  trace_initcall_finish_cb
  initcall_blacklisted
  do_one_initcall
  do_one_initcall
  trace_initcall_start_cb
  run_init_process
  try_to_run_init_process
  match_dev_by_label
  match_dev_by_uuid
  rootfs_init_fs_context
  $

Committer notes:

This is the same command line option and for the same purpose as in
'perf probe'.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
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/20200808023141.14227-3-changbin.du@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>