cmd/internal/obj/loong64: add support for movgr2cf and movcf2gr instructions

cmd/internal/obj/loong64: add support for movgr2cf and movcf2gr instructions

Change-Id: I7ff3c8df24ed7990fe104bc2530354c0bd5fe018
Reviewed-on: https://go-review.googlesource.com/c/go/+/475576
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: xiaodong liu <teaofmoli@gmail.com>
Reviewed-by: WANG Xuerui <git@xen0n.name>