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

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

Go asm syntax:
          VPCNT{B,H,W,V}  VJ, VD
         XVPCNT{B,H,W,V}  XJ, XD

Equivalent platform assembler syntax:
          vpcnt.{b,w,h,d}  vd, vj
         xvpcnt.{b,w,h,d}  xd, xj

Change-Id: Icec4446b1925745bc3a0bc3f6397d862953b9098
Reviewed-on: https://go-review.googlesource.com/c/go/+/620736
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>