target/riscv: Compute mstatus.sd on demand

target/riscv: Compute mstatus.sd on demand

The position of this read-only field is dependent on the current xlen.
Rather than having to compute that difference in many places, compute
it only on read.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20211020031709.359469-16-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>