Set the NX bit for the PML4 recursive mapping

[jtl]

Summary

We shouldn't need to execute code in the recursive page table mappings; therefore, it should be safe to set the NX bit on the PML4E for the recurse page table mappings. According to the Intel docs, the effect of the NX bit should propogate to any page reached through a PML4E which has the NX bit set.