MIPS: Loongson: Convert loongson_halt() to use unreachable()

Use the new unreachable() macro instead of while(1);

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: yanh@lemote.com
Cc: huhb@lemote.com
Cc: zhangfx@lemote.com
Patchwork: http://patchwork.linux-mips.org/patch/823/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>