Object file loading on Windows

The CPU backend is currently broken on windows because it lacks an object file loader ([here](https://github.com/AccelerateHS/accelerate-llvm/blob/master/accelerate-llvm-native/Data/Array/Accelerate/LLVM/Native/Link/COFF.hs)).

This is the same as what GHCi does, but a bit simpler because we only need to handle the most basic types of relocations etc. Unfortunately we can't use that one due to [GHC#13624](https://ghc.haskell.org/trac/ghc/ticket/13624) (unless that does not affect windows, but I doubt it)

Any windows hackers interested in writing one? @RyanGlScott