Add `GC.getWeakRefTarget` function.

A basic functionality for weak reference implementation. We want to be able to do things like this:
```D
if(!atomicLoad(weakPtr)) // fast check
	return null;
return GC.getWeakRefTarget(weakPtr);
```
`lock`/`unlock` pulse feels himself good and thanks for the interest. If the question is about purpose of these lines it's just a `GC` collection fence.