Extend drcpusim to simulate opcodes not present on the underlying hardware

Today drcpusim only simulates non-existence: i.e., you specify a legacy processor and it identifies whether your application uses opcodes not present on that processor.

This is a feature request to also support emulating ISA features not present on the underlying hardware. I.e., you would specify any processor or ISA version and it would fill in any gaps not supported by the current underlying processor.