Add toolchain support for qbs.targetPlatform property

[Psy-Kai]

The Qbs property qbs.targetPlatform defines the os of the build target
(or "none" for bare-metal devices). When implementing the Qbs toolchain I wrongly assumed
that it would be outputted by qbs-setup-toolchains. With this PR the QbsToolchain class will
set the qbs.targetPlatform property based on the value in settings.os.

Changelog: Fix: Set qbs.targetPlatform with qbs toolchain.
Docs: Omit

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^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}

[Psy-Kai]

@rweickelt Would you like to review this too?

[memsharded]

Looking good, @rweickelt wants to do another review?

[rweickelt]

Looks good to me. Thanks for fixing.