[llvm-objdump][NFC] MachODump.cpp interface cleanup

Continuing from D77388, this patch moves interface declarations
associated with `MachODump.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 defined in
`MachODump.cpp` that are not referenced outside of it are given internal
linkage.

This patch does not rename the external functions defined by
`MachODump.cpp` that are not clearly named as being specific to Mach-O.

Reviewed By: jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D77730