target/riscv: add vector index load and store instructions

target/riscv: add vector index load and store instructions

Vector indexed operations add the contents of each element of the
vector offset operand specified by vs2 to the base effective address
to give the effective address of each element.

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: <20200701152549.1218-8-zhiwei_liu@c-sky.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>