[llvm-objdump] [NFC] Factor out DisassemblerTarget class.

This is a preparation for ARM64EC/ARM64X binaries, which may contain both ARM64
and x86_64 code in the same file. llvm-objdump already has partial support for
mixing disassemblers for ARM thumb mode support. However, for ARM64EC we can't
share MCContext, MCInstrAnalysis and PrettyPrinter instances. This patch
provides additional abstraction which makes adding mixed code support later in
the series easier.

Reviewed By: jhenderson, MaskRay
Differential Revision: https://reviews.llvm.org/D149093