[MachO] Fixing ub in MachO BinaryFormat

This isn't encountered anywhere inside LLVM, so I wrote a test case to expose the issue and verify that it is fixed.

The basic problem is that the macho_load_command union contains all load comamnd structs. Load command structs in 32-bit macho files can be 32-bit aligned instead of 64-bit aligned.

There are some strange circumstances in which this can be exposed in a 64-bit macho if the load commands are invalid or if a 32-bit aligned load command is used. In the past we've worked around this type of problem with changes like r264232.

llvm-svn: 333797