32 bit OCaml compiler on 64 bit host

Compilation options taken from ocaml INSTALL file.

Tested on Ubuntu 13.10 64 bit.

This was modified from the base compiler OPAM spec for 4.01.0 which
includes a patch for MacOSX, though I dont expect MacOSX will work.

The only package I have found which doesn't work (I haven't
tested that many...) is `ssl`.  I believe this is because it uses `gcc`
to compile c stubs but doesn't use `-m32`.  A work around was to
install `ssl` with

```
$ CFLAGS="-m32" opam install ssl
```

Note that ssl uses OcamlMakefile to build and other projects which
use C stubs and OcamlMakefile might have the same problem.
add 'for linux' in descr
Merge pull request #1861 from andrewray/compiler32bit

Compiler32bit