- Support single-precision floats as first-class values
- Introduce chunks Many32, Many64 and types Tany32, Tany64 to
  support saving and restoring registers without knowing
  the exact types (int/single/float) of their contents, just
  their sizes.
- Memory model: generalize the opaque encoding of pointers to
  apply to any value, not just pointers, if chunks Many32/Many64
  are selected.
- More properties of FP arithmetic proved.


git-svn-id: https://yquem.inria.fr/compcert/svn/compcert/trunk@2537 fca1b0fc-160b-0410-b1d3-a4f43f01ea2e