[llvm-readobj] Change variable names to match LLVM-style. NFC.

Summary: This patch helps change variable names to match LLVM-style

Reviewers: jhenderson, Higuoxing

Reviewed By: Higuoxing

Subscribers: rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59931

llvm-svn: 357230