Make read_offset also take an IDX_* argument for the referring
If it's a macro, you can make the args short and use IDX_debug_##arg.
read_offset and read_address both need this. They need to know what
section the reference comes from. In the test kludge phase they will
just emit that in their messages to match up with what dwarflint is
looking for. In the real smart reader implementation, they need this
index to look up which relocation section they are handling.