llama : add xcframework build script

[danbev]

This commit adds a script to build an XCFramework for iOS. The script uses CMake to build the library for both the simulator and device and then creates an XCFramework that contains both builds.

The XCFramework can then be added to a project and used in the same way as a regular framework. The llama.swiftui example project has been updated to use the XCFramework and can be started using the following command:

$ open examples/llama.swiftui/llama.swiftui.xcodeproj/

Refs: https://github.com/ggml-org/llama.cpp/issues/10747