Store the boot exception level on arm64 so it can be queried later

A hypervisor, e.g. bhyve, will need to know what exception levelthe kernel
was in when it started booting. If it was EL2 we can then enable said
hypervisor.

Store the boot exception level and allow the kernel to later query it.

Obtained from: https://github.com/FreeBSD-UPB/freebsd (earlier version)
Sponsored by: Innovate UK