fix from upstream for building w/x32

https://code.google.com/p/protobuf/issues/detail?id=474

--- src/google/protobuf/stubs/atomicops.h	(revision 491)
+++ src/google/protobuf/stubs/atomicops.h	(revision 492)
@@ -66,7 +66,7 @@ typedef int32 Atomic32;
 #ifdef GOOGLE_PROTOBUF_ARCH_64_BIT
 // We need to be able to go between Atomic64 and AtomicWord implicitly.  This
 // means Atomic64 and AtomicWord should be the same type on 64-bit.
-#if defined(GOOGLE_PROTOBUF_OS_NACL)
+#if defined(__ILP32__) || defined(GOOGLE_PROTOBUF_OS_NACL)
 // NaCl's intptr_t is not actually 64-bits on 64-bit!
 // http://code.google.com/p/nativeclient/issues/detail?id=1162
 typedef int64 Atomic64;