[llvm-objdump] Create ObjectFile specific dumpers

We pay the one-off boilerplate overhead to create `*Dumper` classes that derive
from objdump::Dumper a la llvm-readobj. This has two primary advantages.

First, a lot object file format specific code can be moved from
llvm-objdump.cpp to *Dump.cpp files. Refactor `printPrivateHeaders` as
an example.

Second, with the introduction of ELFDumper<ELFT>, we can simplify
a few dispatch functions in ELFDump.cpp.

In addition, the ObjectFile specific dumpers contains a ObjectFile specific
reference so that we can remove a lot of `cast<*ObjectFile>(Obj)`.

Reviewed By: mtrofin

Differential Revision: https://reviews.llvm.org/D155045