target/riscv: Add DisasExtend to gen_unary

target/riscv: Add DisasExtend to gen_unary

Use ctx->w for ctpopw, which is the only one that can
re-use the generic algorithm for the narrow operation.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210823195529.560295-12-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>