target/riscv: Introduce altfmt into DisasContext

Reviewed-by: Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com>
Reviewed-by: Chao Liu <chao.liu.zevorn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Max Chou <max.chou@sifive.com>
Message-ID: <20260402125234.1371897-7-max.chou@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>