Add initial support for cross-building to iOS

Tested locally with the following configurations:
- iOS device builds (arm64)
- iOS simulator builds (x86_64)
- iOS simulator_and_device builds (fat arm64 and x86_64 archives)

All iOS builds currently require a custom vcpkg fork which contains
fixes for building the required 3rd party libraries.

qtsvg, qtdeclarative, qtgraphicaleffects and qtquickcontrols2
have also been tested to build successfully.

simulator_and_device builds are also supported, but require an umerged
patch in upstream CMake as well as further patches to vcpkg.

Task-number: QTBUG-75576
Change-Id: Icd29913fbbd52a60e07ea5253fd9c7af7f8ce44c
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: Qt CMake Build Bot
Reviewed-by: Leander Beernaert <leander.beernaert@qt.io>

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