Further followup to Q_OS_MACX changes.

Flip !Q_OS_IOS conditions to Q_OS_MACX where it seems appropriate,
remove a redundant condition in qtextcodec_p.h.

Change-Id: I21c8c0c490f1eb4a9337a7f2f3e907c125489438
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>