target/loongarch: Implement xvmul/xvmuh/xvmulw{ev/od}

This patch includes:
- XVMUL.{B/H/W/D};
- XVMUH.{B/H/W/D}[U];
- XVMULW{EV/OD}.{H.B/W.H/D.W/Q.D}[U];
- XVMULW{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: <20230914022645.1151356-26-gaosong@loongson.cn>