[RFC] gcc-plugins: Add constify plugin

This is a non-operational port of the PaX/Grsecurity constify plugin. The
lack of the pax_open_kernel()/pax_close_kernel() infrastructure and
const_cast() annotations renders this unusable in its current form, but
it's a place to start.

Signed-off-by: Kees Cook <keescook@chromium.org>