target/loongarch: Implement gvec_*_vl functions

Create gvec_*_vl functions in order to hide oprsz.
This is used by gvec_v* functions for oprsz 16,
and will be used by gvec_x* functions for oprsz 32.

Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230914022645.1151356-3-gaosong@loongson.cn>