32-bit ELF file support

[m4b]

This shouldn't be too much work, as goblin already has all the underlying 32-bit struct support, etc.

Do note however this will have to be resolved, I think: https://github.com/m4b/goblin/pull/86#issuecomment-381427027

Other than that, the hard part will be removing hardcoded platform relocations in the backends (which will need to be done for arm, other than x86 architecture support, etc.)