cpu: move empty doinit to own file

Avoids each GOARCH having to define an empty doinit for GOOS=linux

Change-Id: Ic7cea1be4a35c31593c4f867478194b82dd53613
Reviewed-on: https://go-review.googlesource.com/c/sys/+/206859
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cpu/cpu_linux_noinit.go

@@ -0,0 +1,9 @@
+// Copyright 2019 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 linux,!arm,!arm64,!s390x,!ppc64
+
+package cpu
+
+func doinit() {}

cpu/cpu_mips64x.go

@@ -7,5 +7,3 @@
 package cpu
 
 const cacheLineSize = 32
-
-func doinit() {}

cpu/cpu_mipsx.go

@@ -7,5 +7,3 @@
 package cpu
 
 const cacheLineSize = 32
-
-func doinit() {}

cpu/cpu_other_arm64.go

@@ -7,5 +7,3 @@
 package cpu
 
 const cacheLineSize = 64
-
-func doinit() {}

cpu/cpu_wasm.go

@@ -11,5 +11,3 @@ package cpu
 // rules are good enough.
 
 const cacheLineSize = 0
-
-func doinit() {}