arm: mm: add CONFIG_STRICT_MEMORY_RWX

If this is set, kernel text will be made RX, kernel data and stack
RW, rodata R so that writing to kernel text, executing kernel data
or stack, or writing to read-only data or kernel text will not
succeed.

Change-Id: Ib80907b34388bf547c4f268a903a766acaab9ae2
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>
(cherry picked from commit b347b1b5d7e88266f13c971dbd9116826085330c)