Make a few simple changes towards 64-bit iOS support.

Change-Id: Ia931f98f6f155a619b9d3efa5777d0c30e62adac
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@theqtcompany.com>
Qbs: set cpp.platformPath for OS X and iOS profiles.

This is an (effectively) required property which is necessary to locate
a number of SDK tools.

Change-Id: Ia4912e383a053c7abcbd47715cf4b130c4fbafd6
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@theqtcompany.com>