Can't Build Qt for Windows ARM64 with CMake

With qmake it was possible to build for ARM64 using -xplatform win32-arm64-msvc in the configure script. There was special code in a feature file checking for specifically this configuration to make sure that we pretended to be cross compiling (and generating host tools for x64).

With CMake this should work if have an existing host build and configure with -DQT_HOST_PATH="location of x64 build" and then configure with the ARM64 MSVC environment. This configure's fine with cmake, but the first time a host tool is required ("moc"), it tries to use the ARM64 version of "moc" on a x64 host build. What it looks like is that CMake doesn't think this is a cross compilation (because at least in MSVC world it isn't), but that also means that the generated build files don't think this is cross compilation either and doesn't use the host tools.

Windows ARM64 builds are not officially supported, but they did work in 6.0 with qmake. Now that qmake builds of Qt are disabled though it means that CMake needs to support this configuration as well.