target/loongarch: Implement xvmadd/xvmsub/xvmaddw{ev/od}

This patch includes:
- XVMADD.{B/H/W/D};
- XVMSUB.{B/H/W/D};
- XVMADDW{EV/OD}.{H.B/W.H/D.W/Q.D}[U];
- XVMADDW{EV/OD}.{H.BU.B/W.HU.H/D.WU.W/Q.DU.D}.

Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230914022645.1151356-27-gaosong@loongson.cn>