Use general vc environment script instead of platform specific.

Just use the specific scripts to detect whether a toolchain is
installed.
Partial revert of 2be4d264.

Task-number: QTCREATORBUG-16359
Change-Id: Ic8c8fbc30f0c1269aeb51abbd21902097bd6c285
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>