target/riscv: Add J-extension into RISC-V

Signed-off-by: Alexey Baturo <space.monkey.delivers@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 20211025173609.2724490-2-space.monkey.delivers@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>