target/riscv: Use gen_arith for mulh and mulhu

target/riscv: Use gen_arith for mulh and mulhu

Split out gen_mulh and gen_mulhu and use the common helper.

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210823195529.560295-9-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>