Remove explicit type casts from SerializeClosure::do_xxx() calls

We have a few unsightly casts like this in the CDS code:

void CppVtables::serialize(SerializeClosure* soc) {
  soc->do_ptr((void**)&_index);

void StringTable::serialize_shared_table_header(SerializeClosure* soc) {
  ...
  soc->do_u4((u4*)(&_shared_strings_array_root_index));

These type casts can be avoided by adding more member functions to SerializeClosure:

  void do_int(int* p);
  template <typename T> void do_ptr(T** p) { do_ptr((void**)p); }