Fix visionOS build and add CI (llama/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 <[email protected]>

ggml/src/CMakeLists.txt

@@ -325,6 +325,10 @@ if (CMAKE_SYSTEM_NAME MATCHES "Android")
     target_link_libraries(ggml-base PRIVATE dl)
 endif()
 
+if(CMAKE_SYSTEM_NAME MATCHES "visionOS")
+    target_compile_definitions(ggml-base PUBLIC _DARWIN_C_SOURCE)
+endif()
+
 if (BUILD_SHARED_LIBS)
     foreach (target ggml-base ggml)
         set_target_properties(${target} PROPERTIES POSITION_INDEPENDENT_CODE ON)