[Oberon] GC of Extended Oberon now also works if MemLim = N*256

The scan phase of the garbage collector of Extended Oberon now also works if MemLim is a multiple of 256. Thanks to the uses of the system for the input on this over the past few days - it helped arriving at a satisfactory solution quickly. The key is to check for p < heapLim during the scan phase at the right places.