all: add appengine build tag

The purego tag (see https://golang.org/issue/23172) is a community agreed
upon signal that a given build environment does not support unsafe.
The appengine environment is supposed to respect this tag, but does not
properly do so. Add this tag back in until they fix their environment.

Change-Id: I9a70062be4339c2e1a93cac31d387698c561b8aa
Reviewed-on: https://go-review.googlesource.com/c/154743
Reviewed-by: Damien Neil <dneil@google.com>

internal/impl/pointer_reflect.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 purego
+// +build purego appengine
 
 package impl
 

internal/impl/pointer_unsafe.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 !purego
+// +build !purego,!appengine
 
 package impl
 

reflect/protoreflect/value_pure.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 purego
+// +build purego appengine
 
 package protoreflect
 

reflect/protoreflect/value_unsafe.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 !purego
+// +build !purego,!appengine
 
 package protoreflect
 

reflect/prototype/name_pure.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 purego
+// +build purego appengine
 
 package prototype
 

reflect/prototype/name_unsafe.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 !purego
+// +build !purego,!appengine
 
 package prototype