x86: split NX setup into separate file to limit unstack-protected code

Move the NX setup into a separate file so that it can be compiled
without stack-protection while leaving the rest of the mm/init code
protected.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

diff --cc arch/x86/include/asm/pgtable_types.h
Simple merge

diff --cc arch/x86/mm/Makefile
Simple merge

diff --cc arch/x86/mm/init.c
Simple merge

diff --cc arch/x86/mm/setup_nx.c
Simple merge