arm64: Initial SVE support

Add initial kernel support for SVE. This detects if SVE is present on
all CPUs, and if so allows for the use of SVE in the future.

As the SVE registers are a superset of the VFP registers we don't need
to restore the VFP registers when SVE is enabled.

Ths interface to enable SVE is provided, but not used until SVE is
supported in signals and with ptrace.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43306

(cherry picked from commit 332c426328dbb30a6b2e69d9b1e8298d77d85bd1)