defconfig: enable CONFIG_STRICT_MEMORY_RWX by default

Enable CONFIG_STRICT_MEMORY_RWX by default, which
improves security by prohibiting writes to kernel
text, execution of kernel data, etc. Note that this
feature is not being turned on for certain defconfigs
due to memory constraints.

Change-Id: I95defa31cc8194dfeff76f050cfb075c0cf9d95b
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>
Signed-off-by: Neeti Desai <neetid@codeaurora.org>
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>