Add an ocamlbuild rule to build .cmxs files from .cmx

Building a .cmxs using ocamlbuild's standard rule produces a .cmxa
file which contains all needed dependencies (in our case, all Benl
library). Since the latter contains side effects, we want to avoid
linking it all together in plugins that may be used at runtime and
mess around with the runtime state of the program.

This was triggered by bug#876478 where I've found out that loading
a template plugin had as a side-effect of resetting all parameters
to their default values.