Change arm64 simulator register backing store.

src/utils.h

Index: src/utils.h
diff --git a/src/utils.h b/src/utils.h
index eeb098752198d4f1ef6333bf64464e90113f61d7..d5b7274e95eed58e4b17c39006e6dc0ccb99eade 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -233,6 +233,25 @@ inline int32_t WhichPowerOf2Abs(int32_t x) {
 }
 
 
+// Obtains the unsigned type corresponding to T
+// available in C++11 as std::make_unsigned
+template<typename T>
+struct make_unsigned {
+  typedef T type;
+};
+
+
+// Template specializations necessary to have make_unsigned work
+template<> struct make_unsigned<int32_t> {
+  typedef uint32_t type;
+};
+
+
+template<> struct make_unsigned<int64_t> {
+  typedef uint64_t type;
+};
+
+
 // ----------------------------------------------------------------------------
 // BitField is a help template for encoding and decode bitfield with
 // unsigned content.