[llvm-readobj] Replace arch-specific ObjDumper methods by the single `printArchSpecificInfo`

Initially llvm-readobj supports multiple command line options like
`--arm-attributes` and `--mips-plt-got` for display ELF arch-specific
information. Now all these options are superseded by the
`--arch-specific` one. It makes sense to have a single `printArchSpecificInfo`
method in the base `ObjDumper`, and hide all ELF/target specific details
in the `ELFDumper::printArchSpecificInfo` override.

Differential Revision: https://reviews.llvm.org/D68385

llvm-svn: 373731