Respect object format choice on Darwin

Summary:
The object format can be set to something other than MachO, e.g.
to use ELF-on-Darwin for MCJIT. This already works on Windows, so
there's no reason it shouldn't on Darwin.

Reviewers: lhames, grosbach

Subscribers: rafael, grosbach, t.p.northover, llvm-commits

Differential Revision: http://reviews.llvm.org/D6185

llvm-svn: 236455