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 <[email protected]>
Signed-off-by: Sam Ravnborg <[email protected]>

Author: Paolo 'Blaisorblade' Giarrusso