Allow building cpuinfo on Darwin

Signed-off-by: Gregory Eremin <g@erem.in>

Author: localhots

cpuinfo.go

@@ -11,9 +11,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build linux
-// +build linux
-
 package procfs
 
 import (

cpuinfo_darwin.go

@@ -0,0 +1,19 @@
+// Copyright 2020 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build darwin
+// +build darwin
+
+package procfs
+
+var parseCPUInfo = parseCPUInfoDummy