kbuild: fix mode of checkstack.pl and other files.

Make it executable like it should be. Do the same for other files intended to be
executed by the user - the ones called by the build process needn't be
executable as they already work (as argument to their interpreter).

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>