arm64: Support SVE in ptrace and core dumps

Add the NT_ARM_SVE note type and use it to access the SVE registers
from ptrace. This allows userspace to modify the full SVE register
values.

Try to follow the Linux semantics to allow debuggers to use this with
minimal changes.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43309

(cherry picked from commit aab60068943d733b0b4573e5481c543ab3d45a00)