sim: Call regStats of base-class as well

We want to extend the stats of objects hierarchically and thus it is necessary
to register the statistics of the base-class(es), as well.  For now, these are
empty, but generic stats will be added there.

Patch originally provided by Akash Bagdia at ARM Ltd.

src/sim/process.cc

@@ -191,6 +191,8 @@ Process::Process(ProcessParams * params)
 void
 Process::regStats()
 {
+    SimObject::regStats();
+
     using namespace Stats;
 
     num_syscalls

src/sim/system.cc

@@ -393,6 +393,8 @@ System::unserialize(CheckpointIn &cp)
 void
 System::regStats()
 {
+    MemObject::regStats();
+
     for (uint32_t j = 0; j < numWorkIds ; j++) {
         workItemStats[j] = new Stats::Histogram();
         stringstream namestr;