[mach-o] binary reader and writer

This patch adds support for converting normalized mach-o to and from binary
mach-o. It also changes WriterMachO (which previously directly wrote a 
mach-o binary given a set of Atoms) to instead do it in two steps. The first 
step uses normalizedFromAtoms() to convert Atoms to normalized mach-o, and the
second step uses writeBinary() which to generate the mach-o binary file.  

llvm-svn: 194167