bin2s: Mark stack as non-executable.

Every object file linked to supermin has to be marked as having a
non-executable stack.  The object generated by bin2s was not being
marked this way, thus the final supermin binary ended up as having an
executable stack (wrongly).

Before this change:

$ execstack -q src/supermin
X src/supermin

After applying this patch and rebuilding:

$ execstack -q src/supermin
- src/supermin

See also:

http://gcc.gnu.org/ml/gcc-patches/2007-07/msg01155.html
https://wiki.gentoo.org/wiki/Project:Hardened/GNU_stack_quickstart
http://caml.inria.fr/mantis/print_bug_page.php?bug_id=4564