Make llvm-objdump handle both arm and thumb disassembly from the same Mach-O
file with -macho, the Mach-O specific object file parser option.

After some discussion I chose to do this implementation contained in the logic
of llvm-objdump’s MachODump.cpp using a second disassembler for thumb when
needed and with updates mostly contained in the MachOObjectFile class.