Use the execute-never bits on arm64

[andrew]

Summary

Start to use the User and Privileged execute-never bits. 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.

Test Plan

Netboot on a ThunderX to multiuser.

[alc]

sys/arm64/arm64/pmap.c

2433

My impression is that r271716 would also apply to arm64.

[andrew]

Update the check in pmap_protect to be the same as rS271716