LoongArch: Make relocate_new_kernel_size be a .quad value

Now relocate_new_kernel_size is a .long value, which means 32bit, so its
high 32bit is undefined. This causes memcpy((void *)reboot_code_buffer,
relocate_new_kernel, relocate_new_kernel_size) in machine_kexec_prepare()
access out of range memories in some cases, and then end up with an ADE
exception.

So make relocate_new_kernel_size be a .quad value, which means 64bit, to
avoid such errors.

Cc: stable@vger.kernel.org
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>