target/riscv: Fix satp write

These variables should be target_ulong. If truncated to int,
the bool conditions they indicate will be wrong.

As satp is very important for Linux, this bug almost fails every boot.

Signed-off-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210901124539.222868-1-zhiwei_liu@c-sky.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>