internal/subtle: add Google App Engine support

The new package subtle added in golang.org/cl/112236 breaks
compatibility with App Engine due to the import of unsafe.

This changes adds an App Engine alternative without using unsafe.

Tested with:

  $ go test -test.tags=appengine -v
  === RUN   TestAliasing
  --- PASS: TestAliasing (0.00s)
  PASS
  ok  	golang.org/x/crypto/internal/subtle	0.009s

Change-Id: I2fc6b02a860b3ee11fa31652ba302fc7db9df153
Reviewed-on: https://go-review.googlesource.com/119095
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

internal/subtle/aliasing.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !appengine
+
 // Package subtle implements functions that are often useful in cryptographic
 // code but require careful thought to use correctly.
 package subtle // import "golang.org/x/crypto/internal/subtle"