linux_hardened: enable structleak plugin

A port of the PaX structleak plugin.  Note that this version of structleak
seems to cover less ground than the PaX original (only marked structs are
zeroed). [1]

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c61f13eaa1ee17728c41370100d2d45c254ce76f