Add support for byte endian swapping for Word 16/32/64.

* Exposes bSwap{,16,32,64}# primops
* Add a new machops MO_BSwap
* Use a Stg implementation (hs_bswap{16,32,64}) for other implementation
  in NCG.
* Generate bswap in X86 NCG for 32 and 64 bits, and for 16 bits, bswap+shr
  instead of using xchg.
* Generate llvm.bswap intrinsics in llvm codegen.

Patch from Vincent Hanquez.
Fix warnings
Merge branch 'master' of http://darcs.haskell.org/ghc