[llvm-readobj] set --elf-cg-profile as alias of --cg-profile

Summary: Rename --elf-cg-profile to --cg-profile and keep --elf-cg-profile as an alias of --cg-profile.

Reviewers: jhenderson, MaskRay, espindola, hans

Reviewed By: jhenderson, MaskRay

Subscribers: emaste, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81855