target/arm: Adjust aarch64_cpu_dump_state for system mode SVE

Use the existing helpers to determine if (1) the fpu is enabled,
(2) sve state is enabled, and (3) the current sve vector length.

Tested-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20181005175350.30752-6-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>