lucetc does not produce reproducible .so/.dylib files

While we are about to land RLBox support on nightly we should start thinking about the reproducibility story. In fact, we already know that lucetc does not produce reproducible .so/.dylib files. We need to fix that for our alpha transition.

Mozilla's corresponding bug is https://bugzilla.mozilla.org/show_bug.cgi?id=1612035.