runtime, cmd/compile, cmd/internal/obj: remove duff support for loong64

runtime, cmd/compile, cmd/internal/obj: remove duff support for loong64

Change-Id: I44d6452933c8010f7dfbf821a32053f9d1cf151e
Reviewed-on: https://go-review.googlesource.com/c/go/+/700096
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>