csky: add arch support current_stack_pointer

To follow the existing per-arch conventions, using "current_stack_pointer"
to set sp.

This will let it be used in non-arch places(like HARDENED_USERCOPY).

Refer to the implementation of riscv commit fdecfea0 ("riscv: Rename
"sp_in_global" to "current_stack_pointer"").

Link: https://lore.kernel.org/lkml/20220224060411.1855683-1-keescook@chromium.org/


Signed-off-by: Tong Tiangen <tongtiangen@huawei.com>
Signed-off-by: Guo Ren <guoren@kernel.org>