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_test.go

@@ -27,6 +27,8 @@ var aliasingTests = []struct {
 	{a[:], nil, false, false},
 	{nil, nil, false, false},
 	{a[:], a[:0], false, false},
+	{a[:10], a[:10:20], true, false},
+	{a[:10], a[5:10:20], true, true},
 }
 
 func testAliasing(t *testing.T, i int, x, y []byte, anyOverlap, inexactOverlap bool) {