[llvm-readobj] Implement LLVM style printer for --notes

Summary:
Port the GNU style printNotes method to the LLVMStyle subclass.

This is basically just a heavy refactor so that the note parsing/formatting logic from the GNUStyle::printNotes can be shared with LLVMStyle::printNotes.

Reviewers: MaskRay

Reviewed By: MaskRay

Subscribers: dschuff, fedor.sergeev, llvm-commits

Differential Revision: https://reviews.llvm.org/D54220

llvm-svn: 346371