cmd/internal/obj/loong64: add [X]VMULW{EV/OD} series instructions support

cmd/internal/obj/loong64: add [X]VMULW{EV/OD} series instructions support

Go asm syntax:
	 VMULW{EV/OD}{HB/WH/VW/QV}[U]		VK, VJ, VD
	XVMULW{EV/OD}{HB/WH/VW/QV}[U]		XK, XJ, XD
	 VMULW{EV/OD}{HBUB/WHUH/VWUW/QVUV}	VK, VJ, VD
	XVMULW{EV/OD}{HBUB/WHUH/VWUW/QVUV}	XK, XJ, XD

Equivalent platform assembler syntax:
	 vmulw{ev/od}.{h.b/w.h/d.w/q.d}[u]		vd, vj, vk
	xvmulw{ev/od}.{h.b/w.h/d.w/q.d}[u]		xd, xj, xk
	 vmulw{ev/od}.{h.bu.b/w.hu.h/d.wu.w/q.du.d}	vd, vj, vk
	xvmulw{ev/od}.{h.bu.b/w.hu.h/d.wu.w/q.du.d}	xd, xj, xk

Change-Id: Ib1b5fb9605417a2b81841deae40e0e2beb90d03c
Reviewed-on: https://go-review.googlesource.com/c/go/+/658375
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>