[llvm-readobj] - Introduce `forEachRelocationDo` helper.

Our `printStackSize` implementation currently uses
API like `RelocationRef`, `object::symbol_iterator`.
It is not ideal as it doesn't allow
to handle possible error conditions properly.

Some time ago I started rewriting it and this NFC patch is
a one more step toward to it. Here I am introducing the
`forEachRelocationDo` helper. With it it is possible to iterate
over all kinds of relocations, what is helpful for improving
the code in `printStackSize` and around.

Differential revision: https://reviews.llvm.org/D91530