Stack chunk thawing races with concurrent GC stack iteration

In ThawBase::finalize_thaw we do this:

  if (!_stream.is_done()) {
    assert(_stream.sp() >= chunk->sp_address(), "");
    chunk->set_sp(chunk->to_offset(_stream.sp()));
    chunk->set_pc(_stream.pc());
  } else {
    chunk->set_argsize(0);
    chunk->set_sp(chunk->stack_size());
    chunk->set_pc(nullptr);
  }

The sp and argsize are both read when setting up a StackChunkFrameStream from concurrent GC threads to scan the stack. The lack of atomicity implies the GC thread might observe old values of argsize with new values of sp, or vice versa. The consequences are unknown, but this is clearly not right.