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
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Co-authored-by: Giovanni Petrantoni <7008900+sinkingsugar@users.noreply.github.com>