Check ABI

A few caveats with this:

• we currently skip libgfortran and as such don't check whether its ABI was broken;

  this will need work upstream to make it happen: https://sourceware.org/bugzilla/show_bug.cgi?id=23492

• we currently skip libLLVM and libclang and as such don't check whether their ABI was broken;

  checking each of them just takes too much memory (16GB wasn't enough)

  we might be able to tell abidiff to ignore big parts of those libraries, which should help with the memory consumption;

  this could also be improved with some upstream work: https://sourceware.org/bugzilla/show_bug.cgi?id=23596

I think we could merge this as a first step, as it's already better than not checking the ABI at all (at least, if it works correctly 😛). If we do merge this, I'll open issues for the above 2 points so we don't forget about them.

Fixes #108 (closed)