cmd/internal/obj/loong64: remove the incorrect unsigned instructions

cmd/internal/obj/loong64: remove the incorrect unsigned instructions

The loong64 ISA does not support the 32-bit unsigned arithmetic
instructions ADDU, SUBU and MULU.

Change-Id: Ifa67de9c59aa12d08844189ed23e6daad0cc11ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/722760
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>