Re-apply r4159 now with working test on x64

src/platform.h

Index: src/platform.h
diff --git a/src/platform.h b/src/platform.h
index bc2e9d64f3e0af65946987a8b510ebff2f247779..8f6876633626fe4c18e56a77f90148c96edbc318 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -114,6 +114,10 @@ int random();
 namespace v8 {
 namespace internal {
 
+// Use AtomicWord for a machine-sized pointer. It is assumed that
+// reads and writes of naturally aligned values of this type are atomic.
+typedef intptr_t AtomicWord;
+
 class Semaphore;
 
 double ceiling(double x);