UPSTREAM: ARM: mm: allow text and rodata sections to be read-only

This introduces CONFIG_DEBUG_RODATA, making kernel text and rodata
read-only. Additionally, this splits rodata from text so that rodata can
also be NX, which may lead to wasted memory when aligning to SECTION_SIZE.
The read-only areas are made writable during ftrace updates and kexec.

Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Laura Abbott <lauraa@codeaurora.org>
Acked-by: Nicolas Pitre <nico@linaro.org>

(cherry picked from commit 80d6b0c2eed2a504f6740cd1f5ea76dc50abfc4d)
Signed-off-by: Kees Cook <keescook@google.com>
Change-Id: I3ff491ba194e5750b6ee84716e4eeed3dbcf1d7d