Automatically fall back to using native symbols when breakpad symbols are unavailable or outdated

[gabrielesvelto]

It is rather common on local machines to have leftover Breakpad symbols from old builds. Our tests harnesses has no way to tell if the breakpad symbols it finds are outdated or not so it will just instruct fix-stacks to use them which will fail if they are indeed outdated WRT the build. Given fix-stacks knows how to find native symbols automatically we should interpret the --breakpad option as a preference for Breakpad symbols but still fall back to native ones in case they're missing our outdated.