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.

No source files or tests are named. Start by locating drcpusim and tracing its existing legacy-processor opcode checks, then determine how processor or ISA selection and unsupported-feature emulation would fit together. Done means a requested ISA can be simulated when the underlying processor lacks its features, with coverage for the new behavior.

Written by the indexing model from the issue text.