QbsProjectManager: Support generic Unix targets.

The profiles that we currently create do not work with any non-Linux,
non-Mac Unix system.
Note: This patch introduces some additional redundancy to keep the risk
close to zero. Clean-up can be done later in a non-frozen branch.

Task-number: QTCREATORBUG-10968
Change-Id: I4e150d641a726826b8f3bae4b4a25d80c51f08c9
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Reviewed-by: Tim Sander <tim@krieglstein.org>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>