lib/repo: Add some assertions for ABI sizes

lib/repo: Add some assertions for ABI sizes

Things like https://sourceware.org/libabigail/manual/abidiff.html
look interesting but in a brief look I couldn't work out
how to conveniently use them for quick ABI sanity checking without
doing a diff from a previous build (which we could do but would be
more involved).

This way will at least catch struct ABI breaks on x86_64 which
I think we'd be most likely to do accidentally when trying
to use one of the previous unused values.

I found the hole values via gdb's `pahole` command.

Closes: #1108
Approved by: jlebon