ARM: implement basic NX support for kernel lowmem mappings

Add basic NX support for kernel lowmem mappings.  We mark any section
which does not overlap kernel text as non-executable, preventing it
from being used to write code and then execute directly from there.

This does not change the alignment of the sections, so the kernel
image doesn't grow significantly via this change, so we can do this
without needing a config option.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>