target/riscv: Add support for Zacas extension

target/riscv: Add support for Zacas extension

Add support for amocas.w/d/q instructions which are part of the ratified
Zacas extension: https://github.com/riscv/riscv-zacas

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Signed-off-by: Rob Bradford <rbradford@rivosinc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20231207153842.32401-2-rbradford@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>