Loom: Stack chunks allocation code uses TLABs even when TLABs are disabled

Look at this code in `Freeze<ConfigT>::allocate_chunk`:

```
  StackChunkAllocator allocator(klass, size_in_words, stack_size, current);
  oop fast_oop = allocator.try_allocate_in_existing_tlab();
```

...it calls:

```
oop MemAllocator::try_allocate_in_existing_tlab() {
  oop obj = NULL;
  {
    HeapWord* mem = allocate_inside_tlab_fast();
    ...
  }
  return obj;
}

HeapWord* MemAllocator::allocate_inside_tlab_fast() const {
  return _thread->tlab().allocate(_word_size);
}
```

...without checking for `UseTLAB`. If you reach to `_thread->tlab()` when TLABs are disabled, then you would get into the uncomfortable territory of dealing with uninitialized TLABs, because it initialization is predicated on `UseTLAB` itself:

```
void Thread::initialize_tlab() {
  if (UseTLAB) {
    tlab().initialize();
  }
}
```

This also disallows quickly testing the allocation slowpaths in Loom code.