[llvm-objdump][NFC] Declare command-line externs in headers with namespace

Summary:
This patch moves the forward declarations of command-line `cl::*`
externs in `MachODump.cpp` and `llvm-objdump.cpp` into the headers
corresponding to the file that defines the variable. At the same time,
these externs are moved into the `llvm::objdump` namespace. The externs
that are not referenced outside their defining translation unit are made
static.

This does not factor out uses of the Mach-O options from
`llvm-objdump.cpp`.

Reviewers: jhenderson, MaskRay, DiggerLin, jasonliu, daltenty

Reviewed By: jhenderson, MaskRay

Subscribers: rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77388