Pull up following revision(s) (requested by skrll in ticket #666):

sys/arch/arm/arm32/pmap.c: revision 1.443

port-arm/58135: reproducible pmap KASSERT failure for armv7 with NFS root

Don't unconditionally set XN in pmap_clearbit - only set it if a mapping
exists VM_PROT_EXEC is being cleared.

I've simplified the #ifdefs in the patch from the PR.

(martin)