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
recursive 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.

Reviewed by: kib, markj
MFC after: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D14333