Add attach support to drcachesim launcher

To attach DR to an existing process with the drmemtrace client, today the drcachesim launcher cannot be used and the client has to be directly referenced. This works ok for -offline but not for online where the pipe and analyzer have to be coordinated with the client. Plus, it is confusing for -offline to need a separate invocation method. This is a feature request to add attach support to the launcher. Xref https://groups.google.com/g/dynamorio-users/c/HPauJHOe-9w