simd branch ready for review/merge

[Geoffrey Mainland]

The SIMD branch, available as wip/simd, is ready for review/merge. It
could use some review---Simon and Simon, I'd be especially grateful if
you both had a quick look. Some major points:

1) I have added support for AVX 512, although this is necessarily
untested. AVX and AVX2 are also both supported.

2) After the recent churn regarding patching LLVM's GHC calling
convention, by default only 128-bit wide SIMD vectors are passed in
registers, and then only on X86_64. There is a "hidden" flag,
-fllvm-pass-vectors-in-regs, that causes GHC to generate LLVM code that
assumes all vectors are passed in registers by LLVM. This can be used
with a suitably patched version of LLVM, and if we get LLVM 3.4 patched,
we can consider turning it on by default for LLVM 3.4+. This would mean
that we couldn't mix LLVM <3.3-compiled object files with LLVM
>3.4-compiled object files, but I don't see that as much of a problem.

3) utils/genprimcode has been hacked up to allow us to write vector
operations once and have them instantiated at multiple vector types. I'm
not thrilled with this solution, but after discussing with Simon PJ,
what I've implemented seems to be the minimal reasonable solution to the
problem of exploding primop boilerplate. The changes are documented in
compiler/prelude/primops.txt.pp.

4) Error handling is sub-optimal. My patch checks to make sure that
vector primops can be compiled efficiently based on the current set of
dynamic flags. For example, if -mavx is not specified and the user tries
to use a primop that adds together two 256-bit wide vectors of
double-precision elements, the user will see an error message like:

ghc-stage2: sorry! (unimplemented feature or known bug)
  (GHC version 7.7.20130916 for x86_64-unknown-linux):
    256-bit wide floating point SIMD vector instructions require at
least -mavx.

This is because the only good place to check for this kind of error is
during STG->Cmm translation (in compiler/codeGen/StgCmmPrim.hs), and we
don't have much of an error handling infrastructure there in contrast to
when we're working in the typechecking/renaming monad. If there is a
better way/place to do this, please let me know.

Thanks,
Geoff