Target OS and arch can't be specified at the same time as configure_args

We should not be able to configure "Target OS" and "Target arch" in platform configurations at the same time as we configure "Configure arguments". The data in these 2 can contradict.

This results in the situation where Coin's UI shows a cross-compilation whereas the configure argument in reality doesn't tell it to do so. And also vice versa.

The change should be as follows: "If configure arguments are given in qt5.git's platform_configurations, then the -xplatform's argument should be printed as the target platform". It's the mkspec, that could be very uninformative, but that's what we got. We can't make assumptions based on the mkspec, nor do we want that hardcoded in Coin anyway.

After we've fixed that, we can run a commit through qt5.git that removes the "Target OS" and "Target arch" from the yaml files, as Coin fully ignores those values in the GUI as well.

And after all qt5.git branches have been fixed, we'll make a check in Coin that makes sure we don't configure "Target OS" and "Target arch" whenever we have "Configure args" defined in the platform_congurations as well.