OptimizedEntryBlob should extend RuntimeBlob directly

We've had issues with inherited behavior from BufferBlob, such as not forwarding the right header size through the BufferBlob constructor, which led to issues when implementing the optimized upcall support on AArch64.

I think OptimizedEntryBlob should extend RuntimeBlob directly, so that we don't end up accidentally inheriting behaviour like this from BufferBlob that is not appropriate for OptimizedEntryBlob.