Moved the MachOWriter and ELFWriter out of the Target/* files. Placed the

Moved the MachOWriter and ELFWriter out of the Target/* files. Placed the
definition of it into the CodeGen library. This is so that a backend doesn't
necessarily add in these writers if it doesn't use them (like in the lli
program).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@34034 91177308-0d34-0410-b5e6-96231b3b80d8