cpu: add support for detecting cpu features on loong64

Except for lasx, all other features have been implemented
in the Go mainline.

Change-Id: I61a09396ed23d17991b641a1265e952585cb5636
Reviewed-on: https://go-review.googlesource.com/c/sys/+/655355
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: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>

cpu/cpu_test.go

@@ -87,6 +87,14 @@ func TestARM64minimalFeatures(t *testing.T) {
 	}
 }
 
+func TestLOONG64Initialized(t *testing.T) {
+	if runtime.GOARCH == "loong64" {
+		if !cpu.Initialized {
+			t.Fatal("Initialized expected true, got false")
+		}
+	}
+}
+
 func TestMIPS64Initialized(t *testing.T) {
 	if runtime.GOARCH == "mips64" || runtime.GOARCH == "mips64le" {
 		if !cpu.Initialized {