arm64: Don't trap SVE to EL2

As with floating point instructions don't trap SVE instructions to the
hypervisor. This lets us handle then in the kernel.

Reviewed by: imp (earlier version)
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43303

(cherry picked from commit fe5ed2496e44aec018a6215175bba225b20d81fd)