cpu: add a space before build tag comment

Add a space before build tag comments so it corresponds to the format
documented at https://golang.org/pkg/go/build/.

Change-Id: Ia7c724daf70d419dec52a652188799441053a867
Reviewed-on: https://go-review.googlesource.com/c/sys/+/193198
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cpu/cpu_linux.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//+build !amd64,!amd64p32,!386
+// +build !amd64,!amd64p32,!386
 
 package cpu