target/riscv: add vector amo operations

target/riscv: add vector amo operations

Vector AMOs operate as if aq and rl bits were zero on each element
with regard to ordering relative to other instructions in the same hart.
Vector AMOs provide no ordering guarantee between element operations
in the same vector AMO instruction

Signed-off-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200701152549.1218-10-zhiwei_liu@c-sky.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>