third_party/protobuf should build on the x86_64 architecture

third_party/protobuf/src/google/protobuf/stubs/atomicops.h

Index: third_party/protobuf/src/google/protobuf/stubs/atomicops.h
diff --git a/third_party/protobuf/src/google/protobuf/stubs/atomicops.h b/third_party/protobuf/src/google/protobuf/stubs/atomicops.h
index 13156820950512d7896c0874567de6c8189a35be..4721e7debe3268df9c118fd266b4f2b535177316 100644
--- a/third_party/protobuf/src/google/protobuf/stubs/atomicops.h
+++ b/third_party/protobuf/src/google/protobuf/stubs/atomicops.h
@@ -66,7 +66,9 @@ 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(GOOGLE_PROTOBUF_OS_APPLE) || defined(GOOGLE_PROTOBUF_OS_NACL)
+// MacOS uses long for intptr_t and long long for 64-bit types preventing
+// implicit conversions. See <stdint.h>.
 // 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;