selftests/ftrace: Add test cases for entry args at function exit

Add kretprobe and function exit probe test cases for checking whether
those can access entry arguments at function exit correctly.

Link: https://lore.kernel.org/all/170952366504.229804.11605173085475141091.stgit@devnote2/

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>