Wire up primitive support in the assembler backend for writing .o files

Wire up primitive support in the assembler backend for writing .o files
directly on the mac.  This is very early, doesn't support relocations and
has a terrible hack to avoid .machine from being printed, but despite
that it generates an bitwise-identical-to-cctools .o file for stuff like
this:

  define i32 @test() nounwind { ret i32 42 }

I don't plan to continue pushing this forward, but if anyone else was
interested in doing it, it should be really straight-forward.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119136 91177308-0d34-0410-b5e6-96231b3b80d8