sha3: 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: Ic29df4660a0968f58f721ec61943c0b277d4aaad
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/193777
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

sha3/hashes_generic.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 gccgo appengine !s390x
+// +build gccgo appengine !s390x
 
 package sha3
 

sha3/sha3_s390x.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 !gccgo,!appengine
+// +build !gccgo,!appengine
 
 package sha3
 

sha3/sha3_s390x.s

@@ -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 !gccgo,!appengine
+// +build !gccgo,!appengine
 
 #include "textflag.h"
 

sha3/shake_generic.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 gccgo appengine !s390x
+// +build gccgo appengine !s390x
 
 package sha3