Start to use the User and Privileged execute-never bits in the arm64
pagetables. This sets both bits when entering an address we know shouldn't
be executed.

I expect we could mark all userspace pages as Privileged execute-never to
ensure the kernel doesn't branch to one of these addresses.

While here add the ARMv8.1 upper attributes.

Reviewed by: alc, kib (previous version)
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D10360