Add support for linking to xcframeworks

Currently it is impossible to generate -G Xcode project, having dependencies on *.xcframeworks. Neither find_library (with PATH_SUFFIXES xcframework) nor add_library works correctly with xcframeworks. In case of add_library manually specified xcframework just passed to linker as is, which is wrong. Actually Xcode does the next:

builtin-process-xcframework --xcframework /Users/anatoliy.tomilov/Documents/Projects/vulkansdk-macos-1.2.162.1/MoltenVK/MoltenVK.xcframework --platform ios --target-path /Users/anatoliy.tomilov/Documents/Projects/build-project/bin/Debug

The magic behind builtin-process-xcframework Xcode command seems simple: it just select subdirs of xcframework in accordance with selected ARCHS and copies static libraries from them into the build directory. Link command just have: -lMoltenVK argument.