cmd/compile: use generated loops instead of DUFFCOPY on loong64

cmd/compile: use generated loops instead of DUFFCOPY on loong64

Change-Id: If9da2b5681e5d05d7c3d51f003f1fe662d3feaec
Reviewed-on: https://go-review.googlesource.com/c/go/+/699855
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>