inorder: implement separate fetch unit

instead of having one cache-unit class be responsible for both data and code
accesses, separate code that is just for fetch in it's own derived class off the
original base class. This makes the code easier to manage as well as handle
future cases of special fetch handling

src/cpu/inorder/SConscript

@@ -75,6 +75,7 @@ if 'InOrderCPU' in env['CPU_MODELS']:
         Source('resources/bpred_unit.cc')
         Source('resources/branch_predictor.cc')
         Source('resources/cache_unit.cc')
+        Source('resources/fetch_unit.cc')	
         Source('resources/use_def.cc')
         Source('resources/decode_unit.cc')
         Source('resources/inst_buffer.cc')