Allow mixing of thread offsets between 32 and 64bit architectures.

Begin a more full implementation x86-64 REX prefixes.
Doesn't implement 64bit thread offset support for the JNI compiler.

Change-Id: If9af2f08a1833c21ddb4b4077f9b03add1a05147
diff --git a/runtime/offsets.h b/runtime/offsets.h
index ed4e49e..72a6b0f 100644
--- a/runtime/offsets.h
+++ b/runtime/offsets.h
@@ -50,6 +50,7 @@
 };
 
 // Offsets relative to the current running thread.
+template<size_t pointer_size>
 class ThreadOffset : public Offset {
  public:
   explicit ThreadOffset(size_t val) : Offset(val) {}