cpu: new package

This CL introduces a new cpu package for CPU/platform feature detection.
The cpu package is basically a copy of `internal/cpu` of the standard library.
Revision: bf86aec25972f3a100c3aa58a6abcbcc35bdea49

This CL does not export ARM64 and PPC64 feature detection since at the moment
ARM64/PPC64 requires standard library support.

Updates golang/go#24843

Change-Id: I11bc1ca60b116e902c941b5887c00870dbb1f899
Reviewed-on: https://go-review.googlesource.com/107015
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cpu/cpu_test.go

@@ -0,0 +1,28 @@
+// Copyright 2018 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_test
+
+import (
+	"runtime"
+	"testing"
+
+	"golang.org/x/sys/cpu"
+)
+
+func TestAMD64minimalFeatures(t *testing.T) {
+	if runtime.GOARCH == "amd64" {
+		if !cpu.X86.HasSSE2 {
+			t.Fatalf("HasSSE2 expected true, got false")
+		}
+	}
+}
+
+func TestAVX2hasAVX(t *testing.T) {
+	if runtime.GOARCH == "amd64" {
+		if cpu.X86.HasAVX2 && !cpu.X86.HasAVX {
+			t.Fatalf("HasAVX expected true, got false")
+		}
+	}
+}