Add a write barrier for TVAR closures

This improves GC performance when there are a lot of TVars in the
heap.  For instance, a TChan with a lot of elements causes a massive
GC drag without this patch.

There's more to do - several other STM closure types don't have write
barriers, so GC performance when there are a lot of threads blocked on
STM isn't great.  But fixing the problem for TVar is a good start.