ProjectExplorer: Detect all msvc toolchains.

The host system will not be taken into account, so the cross build
toolchains are also detected.

Task-number: QTCREATORBUG-10998
Change-Id: I04a410bbf368898b70d9560d2c2a4a6c60ed0b61
Reviewed-by: Daniel Teske <daniel.teske@digia.com>