cpu: add mips64x feature detection

Follow CL 200579

Change-Id: Ibedd6569bda3d7836ccb77a7746ed0e5df13c633
Reviewed-on: https://go-review.googlesource.com/c/sys/+/221698
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

cpu/cpu_linux_mips64x.go

@@ -0,0 +1,22 @@
+// Copyright 2020 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.
+
+// +build mips64 mips64le
+
+package cpu
+
+// HWCAP bits. These are exposed by the Linux kernel 5.4.
+const (
+	// CPU features
+	hwcap_MIPS_MSA = 1 << 1
+)
+
+func doinit() {
+	// HWCAP feature bits
+	MIPS64X.HasMSA = isSet(hwCap, hwcap_MIPS_MSA)
+}
+
+func isSet(hwc uint, value uint) bool {
+	return hwc&value != 0
+}