make existing ictce toolchains compatible with HierarchicalMNS

following the example of #1014, see https://github.com/hpcugent/easybuild-framework/issues/980 and https://github.com/hpcugent/easybuild-framework/issues/986

Although this introduces two extra modules (`iccifort` and `iimpi`), it does not break backward compatibility with the previous `ictce` definitions.

@wpoely86: please review?