riscv: u64ilp32: Add xlen_t in user_regs_struct

The u64ilp32 xlen is 64-bit, not the size of long, so change the
elements of user_regs_struct with xlen_t to match different
__riscv_xlen.

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