ftrace: add a script to produce a hierarchical view of a function trace

This script parses a function trace and then produces a hierarchical
view of the function call stack after processing it into a tree.

Changes on V2 thanks to the trace sent by Steven:

- Support both the files "trace" and "trace_pipe" (comments and space
  differences)

- Correct the mini HOW-TO at the beginning.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>