cmd/internal/obj/loong64: add {,X}VEXTRINS.{B,H,W,V} instruction support

cmd/internal/obj/loong64: add {,X}VEXTRINS.{B,H,W,V} instruction support

Go asm syntax:
	 VEXTRINS{B,H,W,V}	$0x1b, vj,vd
	XVEXTRINS{B,H,W,V}	$0x1b, vj,vd

Equivalent platform assembler syntax:
         vextrins.{b,h,w,d}     vd, vj, $0x1b
        xvextrins.{b,h,w,d}     xd, xj, $0x1b

Change-Id: Ibc0bf926befaa2f810cfedd9a40f7ad9a6a9d7fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/716803
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: Michael Knyszek <mknyszek@google.com>