arm64: Add arm64 SVE tests

[andrew]

Summary

Add the tests/sys/arch directory for architecture-specific tests and
use it to add arm64 SVE tests. These test the kernel is managing the
SVE state in a way we expect.

These tests require SVE hardware support to run so will skip when they
can't detect it.

Sponsored by: Arm Ltd

[imp]

So would you also need a getcontext/setcontext test with SVE state changing in between test too?

[andrew]

Needs an update to the appropriate mtree file