[llvm-readobj/elf] - Generalize the code for printing dynamic relocations. NFCI.

Currently we have 2 large `printDynamicRelocations` methods that
have a very similar code for GNU/LLVM styles.

This patch removes the duplication and renames them to `printDynamicReloc`
for consistency.

Differential revision: https://reviews.llvm.org/D87087