[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.  



git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@194167 91177308-0d34-0410-b5e6-96231b3b80d8