cmd/internal/obj/loong64: add {V,XV}SEQ.{B,H,W,D} instructions support

cmd/internal/obj/loong64: add {V,XV}SEQ.{B,H,W,D} instructions support

Go asm syntax:
         VSEQ{B,H,W,V}  VJ, VK, VD
        XVSEQ{B,H,W,V}  XJ, XK, XD

Equivalent platform assembler syntax:
         vseq.{b,w,h,d}  vd, vj, vk
        xvseq.{b,w,h,d}  xd, xj, xk

Change-Id: Ia87277b12c817ebc41a46f4c3d09f4b76995ff2f
Reviewed-on: https://go-review.googlesource.com/c/go/+/616076
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Qiqi Huang <huangqiqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
Reviewed-by: Cherry Mui <cherryyz@google.com>