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

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

Go asm syntax:
	 VNEG{B/H/W/V}		VJ, VD
	XVNEG{B/H/W/V}		XJ, XD

Equivalent platform assembler syntax:
	 vneg.{b/h/w/d}		vd, vj
	xvneg.{b/h/w/d}		xd, xj

Change-Id: Ie0a82a434b0ffbcb77425a65b96eff56e030028c
Reviewed-on: https://go-review.googlesource.com/c/go/+/635935
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>