CheckerCPU: Re-factor CheckerCPU to be compatible with current gem5

Brings the CheckerCPU back to life to allow FS and SE checking of the
O3CPU.  These changes have only been tested with the ARM ISA.  Other
ISAs potentially require modification.

src/cpu/SConscript

@@ -137,7 +137,9 @@ if env['FULL_SYSTEM']:
         Source('legiontrace.cc')
 
 if env['USE_CHECKER']:
+    SimObject('DummyChecker.py')
     Source('checker/cpu.cc')
+    Source('dummy_checker_builder.cc')
     DebugFlag('Checker')
     checker_supports = False
     for i in CheckerSupportedCPUList: