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);
```
This (and I haven't tried to figure out exactly how it is supposed to work yet) definitely needs an explanatory comment in the code.