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_linux_loong64.go

@@ -0,0 +1,22 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cpu
+
+// HWCAP bits. These are exposed by the Linux kernel.
+const (
+	hwcap_LOONGARCH_LSX  = 1 << 4
+	hwcap_LOONGARCH_LASX = 1 << 5
+)
+
+func doinit() {
+	// TODO: Features that require kernel support like LSX and LASX can
+	// be detected here once needed in std library or by the compiler.
+	Loong64.HasLSX = hwcIsSet(hwCap, hwcap_LOONGARCH_LSX)
+	Loong64.HasLASX = hwcIsSet(hwCap, hwcap_LOONGARCH_LASX)
+}
+
+func hwcIsSet(hwc uint, val uint) bool {
+	return hwc&val != 0
+}