Return a `CDataModel` for `CalllingConvention::AppleAarch64`

[Spartan2909]

According to Apple's documentation, their C data model seems to conform to LP64.

This Wikipedia article also says that macOS uses LP64.

Unless I've missed something, it should be possible to simply return CDataModel::LP64 from Triple::data_model when the calling convention is AppleAarch64.

I'm happy to open a PR for this if this is the right solution.