Support reading from the arm64 ID registers from userspace.

Trap reads to the arm64 ID registers and write a safe value into them. This
will allow us to put more useful values in these later and have userland
check them to find what features the hardware supports.

These are currently safe defaults, but will later be populated with better
values from the hardware.

Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D16533