cmd/internal/obj/loong64: add aliases to 32-bit arithmetic instructions

cmd/internal/obj/loong64: add aliases to 32-bit arithmetic instructions

Both the MULW and MUL instructions point to the mul.w instruction
in the loong64 ISA. Previously, MULW was not encoded; now it is
encoded and used as an alias for MUL.
The same applies to the following instructions: ADD, SUB, DIV.
For consistency, we have added additional aliases for DIVU, REM and REMU.

Change-Id: Iba201a3c4c2893ff7d301ef877fad9c81e54291b
Reviewed-on: https://go-review.googlesource.com/c/go/+/721523
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>