Migrate the roundtrip tests to work similar to the llgdb tests

I'm not sure if there's a way to make this a bit more general - the
property that matters is that there's /some/ itanium ABI target the
tests can use to compile - not link or run in this case. But this seems
sufficient for the llgdb tests, so it should be sufficient (though
perhaps not necessary) for this roundtrip test.