riscv: s64ilp32: Add sbi support

The sbi uses xlen as base argument elements to connect m-mode and
s-mode. The previous implementation assumes sizeof(xlen_t) =
sizeof(long), but the s64ilp32's are different. So modify the sbi code
suitable with the s64ilp32 change.

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