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/compiler/driver/compiler_driver.h b/compiler/driver/compiler_driver.h
index 256aa46..ddb62e1 100644
--- a/compiler/driver/compiler_driver.h
+++ b/compiler/driver/compiler_driver.h
@@ -719,6 +719,7 @@
 
   const InstructionSet instruction_set_;
   const InstructionSetFeatures instruction_set_features_;
+  const bool instruction_set_is_64_bit_;
 
   // All class references that require
   mutable ReaderWriterMutex freezing_constructor_lock_ DEFAULT_MUTEX_ACQUIRED_AFTER;