[llvm-readobj] Implement --dependent-libraries for GNU output

Previously, the option was only implemented for LLVM output. This fixes
https://bugs.llvm.org/show_bug.cgi?id=45695.

At the current time, GNU readelf does not support this option.
Consequently, this patch simply attempts to roughly follow the output
style for similar options like --syms/--notes etc, combined with
--string-dump output.

Reviewed by: MaskRay, grimar

Differential Revision: https://reviews.llvm.org/D79939