[llvm-readobj] Refactor ELFDumper::printAttributes()

The current implementation of printAttributes makes it fiddly to extend
attribute support for new targets.

By refactoring the code so all target specific variables are
initialized in a switch/case statement, it becomes simpler to extend
attribute support for new targets.

Reviewed By: jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D107968