cmd/internal/obj/loong64: add MULW.D.W[U] instructions

cmd/internal/obj/loong64: add MULW.D.W[U] instructions

Go asm syntax:
	MULWVW		RK, RJ, RD
	MULWVWU		RK, RJ, RD

Equivalent platform assembler syntax:
	mulw.d.w	rd, rj, rk
	mulw.d.wu	rd, rj, rk

Change-Id: Ie46a21904a4c25d04200b0663f83072c38a76c6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/721521
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Keith Randall <khr@google.com>