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>
diff --git a/cpu/cpu_linux_noinit.go b/cpu/cpu_linux_noinit.go
new file mode 100644
index 0000000..192198e
--- /dev/null
+++ b/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() {}