If a CPU advertises the NX bit in its /proc/cpuinfo flags, we should always pick
the SMP kernel, as we need a PAE enabled kernel to take advantage of it (which
the UP kernels don't have).

This is useful for newer 32 bit VIA CPUs, and newer Intel boxes with HT disabled
running a 32 bit OS.

Otherwise, execshield falls back to using segment limits to approximate NX,
which is kind of a waste.