Use the pre-packaged toolchain provided by Loongson via github.

Tested-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220704070824.965429-1-richard.henderson@linaro.org>