cmd/internal/obj/loong64: add {V,XV}MUL{B/H/W/V} and {V,XV}MUH{B/H/W/V}[U] instructio...

cmd/internal/obj/loong64: add {V,XV}MUL{B/H/W/V} and {V,XV}MUH{B/H/W/V}[U] instructions support

Go asm syntax:
	 VMUL{B/H/W/V}		VK, VJ, VD
	 VMUH{B/H/W/V}[U]	VK, VJ, VD
	XVMUL{B/H/W/V}		XK, XJ, XD
	XVMUH{B/H/W/V}[U]	XK, XJ, XD

Equivalent platform assembler syntax:
	 vmul.{b/h/w/d}		vd, vj, vk
	 vmuh.{b/h/w/d}[u]	vd, vj, vk
	xvmul.{b/h/w/d}		xd, xj, xk
	xvmuh.{b/h/w/d}[u]	xd, xj, xk

Change-Id: I2f15a5b4b6303a0f82cb85114477f58e1b5fd950
Reviewed-on: https://go-review.googlesource.com/c/go/+/636375
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>