target/loongarch: Implement xvadd/xvsub

This patch includes:
- XVADD.{B/H/W/D/Q};
- XVSUB.{B/H/W/D/Q}.

Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230914022645.1151356-15-gaosong@loongson.cn>