[llvm-readobj] - Refactor printGroupSections methods.

This refactors GNUStyle<ELFT>::printGroupSections and
LLVMStyle<ELFT>::printGroupSections to split out all
duplicated code.

After the change these methods just prints the data provided
by introduced getGroups in a corresponding LLVM/GNU format.

Differential revision: https://reviews.llvm.org/D37621

llvm-svn: 313234