[PATCH v2] ARM: mm: implement CONFIG_DEBUG_RODATA

[Kees Cook]

This implements CONFIG_DEBUG_RODATA, along with ftrace support. It is
intended to be used on top of Rabin Vincent's patch "arm: ftrace: work
with CONFIG_DEBUG_SET_MODULE_RONX", so that ftrace will work with both
config options.

Thanks!

-Kees