riscv: s64ilp32: Unify ULL & UL into UXL in csr

The 64ilp32's long size has been different from the csr xlen, so
introduce a new macro of UXL to distinguish UL & ULL.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>