use QT_TARGET_ARCH to identify target architecture

83f473b changed the semantics of QT_ARCH to be the arch of the currently
built target, not that of the qt libraries. we need the latter here.
on the way, also remove the pointless fallbacks to the host arch.

Change-Id: Ia8c742a5aef36b70a182e6dbad97e41ce7ecbf6e
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Wolfgang Bremer <wbremer@blackberry.com>
Reviewed-by: Bernd Weimer <bweimer@blackberry.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>