ftrace/x86: Do not trace .discard.text section

The section called .discard.text has tracing attached to it and is
currently ignored by ftrace. But it does include a call to the mcount
stub. Adding a notrace to the code keeps gcc from adding the useless
mcount caller to it.

Link: http://lkml.kernel.org/r/20110421023739.243651696@goodmis.org


Signed-off-by: Steven Rostedt <rostedt@goodmis.org>