ber_metaocaml: cleanup, tweaks

This puts MetaOCaml on more equal footing with the normal OCaml
packages, which have a few passthru's and expect to have more 'meta'
information available (such as 'platforms').

With these changes, you can use the ber_metaocaml package along with
ocaml-ng.mkOcamlPackages in order to create a full package set with
Native MetaOCaml support, though this currently isn't implemented (you
have to do this yourself).

There are also other light cleanups, for example this also removes the
old MIPS support and restricts the platforms to x86 Linux/Darwin, for
now. Other platforms can be added on a case-by-case basis.

Signed-off-by: Austin Seipp <aseipp@pobox.com>