CMake: Fix default architecture selection for macOS and iOS

Before this change, we created a CMake toolchain file for iOS and
macOS universal which propagated the initially configured
CMAKE_OSX_ARCHITECTURES values to user projects.

So if Qt was configured with 2 arches, configuring a CMake user
project using the generated toolchain file would also build the
user project targeting those 2 arches.

The reason for this that the same toolchain file is used for
configuring both Qt and users projects and we needed to ensure that
other Qt repos are built with the same set of arches. That
unfortunately led the multiple arches to carry over into user
projects.
This is different from qmake behavior which configured user projects
with 1 arch only.

Instead of the toolchain file explicitly setting
CMAKE_OSX_ARCHITECTURES for all projects, save the initial list of
arches into QT_OSX_ARCHITECTURES.
Then if the toolchain file detects that we're building a Qt repo (by
checking for the presence of QT_REPO_MODULE_VERSION) set
CMAKE_OSX_ARCHITECTURES to QT_OSX_ARCHITECTURES to propagate the
initial list of arches.

For user projects we want to have some sensible defaults.
For macOS projects, leave the decision of the architecture to build
to CMake.
For iOS Xcode projects, leave the decision to Xcode.
For iOS Ninja projects, set the architecture to the first value of the
architectures used when configuring Qt.

As a side note this fixes an issue in our CI where we configured macOS
Qt with 2 architectures and then tried to run CMake build tests for
both architectures on a machine that doesn't have the universal SDK.
This led to build failures.
Because the CMake build tests act as regular user projects, now they
are configured with a single architecture that is automtically
detected by CMake.

Pick-to: 6.1
Task-number: QTBUG-85447
Change-Id: Id1b7e78d7e67c1796efed10751416e5f857c16d2
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

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