LoongArch has implemented the current_stack_pointer macro, so select
ARCH_HAS_CURRENT_STACK_POINTER in Kconfig. This will let it be used in
non-arch places (like HARDENED_USERCOPY).
Reviewed-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>