Relocatable binaries not handled

[infinisil]

Describe the bug

Relocatable binaries aren't processed and give an error.

Offending code: https://github.com/NixOS/patchelf/blob/917ea45b79de04f69059f42a8e2621f7caeae1c9/src/patchelf.cc#L281-L282

Missing handling of ET_REL (see ELF headers)

A clear and concise description of what the bug is.

Steps To Reproduce

See: https://hydra.nixos.org/build/241099651/nixlog/1

Expected behavior

Maybe they should also be processed, but I'm not sure

patchelf --version output

patchelf 0.15.0

Additional context

Found by @prtzl and @fricklerhandwerk at #ZurichZHF :)