ARM: mm: allow non-text sections to be non-executable

Adds CONFIG_ARM_KERNMEM_PERMS to separate the kernel memory regions
into section-sized areas that can have different permisions. Performs
the NX permission changes during free_initmem, so that init memory can be
reclaimed.

This uses section size instead of PMD size to reduce memory lost to
padding on non-LPAE systems.

Based on work by Brad Spengler, Larry Bassel, and Laura Abbott.

Change-Id: I581132df1b77156675d421eae0f3f2f53d72ca05
Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Laura Abbott <lauraa@codeaurora.org>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>
Git-commit: 1e6b48116a95046ec51f3d40f83aff8b006674d7
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[shashim@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>