[llvm-readobj/elf] - Introduce Relocation<ELFT> helper.

It removes templating for Elf_Rel[a] handling that we
introduced earlier and introduces a helper class instead.

It was briefly discussed in D87087, which showed,
why having templates is probably not ideal for the generalization
of dumpers code.

Differential revision: https://reviews.llvm.org/D87141