[llvm-objdump][ELF][NFC] Create ELFDump.h

Summary:
Continuing from D77285, the external interfaces implemented by
`ELFDump.cpp` are now declared in `ELFDump.h` and moved into the
`llvm::objdump` namespace. Externs defined in `ELFDump.cpp` that are
unreferenced externally are also made static.

Reviewers: jhenderson, MaskRay, DiggerLin, jasonliu, daltenty

Reviewed By: jhenderson, MaskRay

Subscribers: RKSimon, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78695