all: delete loong64 non-register ABI fallback path

all: delete loong64 non-register ABI fallback path

Change-Id: If1d3eba9a922ac6f9d78301bb8f07e445c712899
Reviewed-on: https://go-review.googlesource.com/c/go/+/525576
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Commit-Queue: abner chenc <chenguoqi@loongson.cn>
Run-TryBot: abner chenc <chenguoqi@loongson.cn>