all: add a space before +build in build tag comments

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

Change-Id: I8349d0343597e304b97fb5479847231ed8945b1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/193237
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/modload/testgo.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 testgo
+// +build testgo
 
 package modload
 

src/cmd/go/internal/renameio/renameio_test.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 !plan9
+// +build !plan9
 
 package renameio
 

src/cmd/go/internal/renameio/umask_test.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 !nacl,!plan9,!windows,!js
+// +build !nacl,!plan9,!windows,!js
 
 package renameio
 

src/cmd/go/internal/robustio/robustio_other.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 !windows
+// +build !windows
 
 package robustio
 

src/cmd/go/internal/web/url_other.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 !windows
+// +build !windows
 
 package web
 

src/cmd/go/internal/web/url_other_test.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 !windows
+// +build !windows
 
 package web
 

src/cmd/go/testdata/script/test_compile_binary.txt

@@ -4,5 +4,5 @@ env GO111MODULE=off
 stderr 'build comment'
 
 -- compile_binary/foo_test.go --
-//+build foo
+// +build foo
 package foo

src/runtime/semasleep_test.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 !nacl,!plan9,!windows,!js
+// +build !nacl,!plan9,!windows,!js
 
 package runtime_test