8027675: Full collections with Serial slower in JDK 8 compared to 7u40

Reduced the number of calls to follow_class_loader and instead marked and pushed the klass holder directly. Also removed unneeded calls to adjust_klass.

Reviewed-by: coleenp, jmasa, mgerdin, tschatzl

hotspot/src/share/vm/oops/instanceKlass.cpp

@@ -2157,7 +2157,6 @@ int InstanceKlass::oop_adjust_pointers(oop obj) {
     obj, \
     MarkSweep::adjust_pointer(p), \
     assert_is_in)
-  MarkSweep::adjust_klass(obj->klass());
   return size;
 }