Fix visionOS build and add CI (#12415) * ci: add visionOS build workflow Add a new GitHub Actions workflow for building on visionOS with CMake and Xcode. * ggml: Define _DARWIN_C_SOURCE for visionOS to fix missing u_xxx typedefs * ci: remove define hacks for u_xxx system types --------- Co-authored-by: Giovanni Petrantoni <7008900+sinkingsugar@users.noreply.github.com>