target/riscv: Use extracts for sraiw and srliw

target/riscv: Use extracts for sraiw and srliw

These operations can be done in one instruction on some hosts.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210823195529.560295-14-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>