Create new architectural flags for the no execute bit for x86 and Generic offset for MMU structures

In porting the vmm of LK to x86-64 architecture we found we need a new ARCH flag for the NX bit present on the platform, also the offset for the MMU structures are different for 64 bit architectures, we proposed this change to be able to be portable along different words sizes, let me know if that is ok

Thanks
Sergio