target/riscv: reformat @sh format encoding for B-extension

target/riscv: reformat @sh format encoding for B-extension

Signed-off-by: Kito Cheng <kito.cheng@sifive.com>
Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210505160620.15723-2-frank.chang@sifive.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>