ftrace/recordmcount: Modify only executable sections

PROGBITS is not enough to determine if the section should be modified
or not. Only process sections that are marked as executable.

Cc: John Reiser <jreiser@bitwagon.com>
Link: http://lkml.kernel.org/r/20110421023737.991485123@goodmis.org


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