DebugInfo: Avoid truncating addr_base to 32 bits

I'm /guessing/ this isn't terribly testable without a very large input
file. Even generated from a more compact assembly file, it's probably
best not to generate a giant temporary test file - if I'm wrong about
that/anyone has good suggestions for testing, I'm all ears!

Based on post-commit review feedback from Igor Kudrin on
eed02423