Exclude assembler from app engine build.

Fixes #7
diff --git a/crc32_amd64.go b/crc32_amd64.go
index 42894ba..2766f96 100644
--- a/crc32_amd64.go
+++ b/crc32_amd64.go
@@ -1,3 +1,5 @@
+//+build !appengine
+
 // Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
diff --git a/crc32_amd64p32.go b/crc32_amd64p32.go
index 2e70a8e..5145398 100644
--- a/crc32_amd64p32.go
+++ b/crc32_amd64p32.go
@@ -1,3 +1,5 @@
+//+build !appengine
+
 // Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
diff --git a/crc32_generic.go b/crc32_generic.go
index aba2fa8..d6f8f85 100644
--- a/crc32_generic.go
+++ b/crc32_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 386 arm arm64 ppc64 ppc64le
+// +build 386 arm arm64 ppc64 ppc64le appengine
 
 package crc32