Add runtime option to disable start/stop API and annotations and then remove the ifdefs

For running untrusted applications, it might be nice to disable all cases where DR listens to the app for what to do: the start/stop API and annotations.

Today we have ifdefs for both of those: but we'd like to remove the DR_APP_EXPORTS define (xref #2971), and the ANNOTATIONS define for that matter once it's ported to AArchXX. Plus it is harder to maintain a separate build.

This option proposes adding a runtime option to disable these features and adding a test to the CI for this option. I think the overhead of checking the option in the inner interp loop will be worth the code cleanliness and maintenance gain?