gcc-plugins: Add constify plugin

This is a port of the PaX/grsecurity constify plugin. However, it has
the automatic function-pointer struct detection temporarily disabled. As
a result, this will only recognize the __do_const annotation, which
makes all instances of a marked structure read-only. The rare_write()
infrastructure can be used to make changes to such variables.

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