arm64: Enable SVE in userspace
Report when SVE is present and allow it to be used by calling
sve_restore_state on an SVE exception from userspace.
Reviewed by: kib
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43310
(cherry picked from commit 205c1007be6d57b2eb8620f79384cb2299dde40e)