arm64: Add an SVE sysarch

To allow for user space to read the SVE vector length add a sysarch
handler to return the value to userspace.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43308

(cherry picked from commit 87a58d931db7b40d8d225a7edb82b78937078b7f)