perf ftrace: Add support for tracing option 'func_stack_trace'

This adds support to display call trace for function tracer. To do this,
just specify a '--func-opts call-graph' option.

Example:

  $ sudo perf ftrace -T vfs_read --func-opts call-graph
   iio-sensor-prox-855   [003]   6168.369657: vfs_read <-ksys_read
   iio-sensor-prox-855   [003]   6168.369677: <stack trace>
   => vfs_read
   => ksys_read
   => __x64_sys_read
   => do_syscall_64
   => entry_SYSCALL_64_after_hwframe
   ...

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/20200808023141.14227-9-changbin.du@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-ftrace.txt

@@ -76,6 +76,10 @@ OPTIONS
 	specify multiple functions (or glob patterns).  It will be
 	passed to 'set_ftrace_notrace' in tracefs.
 
+--func-opts::
+	List of options allowed to set:
+	  call-graph - Display kernel stack trace for function tracer.
+
 -G::
 --graph-funcs=::
 	Select function_graph tracer and set graph filter on the given