cpu: add LLACQ_SCREL, SCQ, DBAR_HINTS detection for loong64

These three feature detections have been implemented in the Go mainline.

Change-Id: I9c264f7dac0630931d851c04f904d4365d70a28b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/779781
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: abner chenc <chenguoqi@loongson.cn>