Make llvm-objdump handle both arm and thumb disassembly from the same Mach-O

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215931 91177308-0d34-0410-b5e6-96231b3b80d8