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Add new target `clean-external` to the Makefile, which triggers the
clean task of the respective buid system for each external simulator.
Each simulator can also be cleaned individually by using the targets
`{simulator name}-clean` (e.g. `gem5-clean`).

`make distclean` now actually removes the external simulators

this gets rid of the non-fatal build error 'No names found, cannot describe anything'

Simply installs vanilla simics under sims/external/simics. Because of licensing
restrictions we cannot automatically download simics from the intel page.
Instead the makefile will prompt the user to download to files and add them to
the simics directory.

This commit does not include any simbricks integration at all yet.
Co-authored-by: Antoine Kaufmann <antoinek@mpi-sws.org>

But this time without the broken whitespace