target/riscv: rvb: support and turn on B-extension from command line

target/riscv: rvb: support and turn on B-extension from command line

B-extension is default off, use cpu rv32 or rv64 with x-b=true to
enable B-extension.

Signed-off-by: Kito Cheng <kito.cheng@sifive.com>
Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210505160620.15723-17-frank.chang@sifive.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>