Split out the arm64 EL2 exception vectors

These were originally in locore.S as they are only needed so we have
a valid value to put into the vbar_el2 register. As these will soon
be used by bhyve so move them to a new file as we already have with
the EL1 exception vectors in exception.S.

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