MC/Mach-O: Introduce Object/MachOFormat for describing purely platform / machine

independent information on the Mach object format, and move some stuff from
MachObjectWriter.cpp there.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120186 91177308-0d34-0410-b5e6-96231b3b80d8