target/loongarch: Introduce abstract TYPE_LOONGARCH64_CPU

In preparation of introducing TYPE_LOONGARCH32_CPU, introduce
an abstract TYPE_LOONGARCH64_CPU.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230821125959.28666-5-philmd@linaro.org>
Signed-off-by: Song Gao <gaosong@loongson.cn>
2 files changed