libretro: support building for 64-bit iOS devices (arm64)

[yoshisuga]

No description provided.

[jet082]

I can confirm this compiles fine. However, it does not copy correctly. Below is the actual location of the file.

** BUILD SUCCESSFUL! GG NO RE **
cp "mesens_libretro_ios.dylib" "/Users/chucksmith/retroarch-stuff/libretro-super/dist/ios-arm64/mesens_libretro_ios.dylib"
cp: mesens_libretro_ios.dylib: No such file or directory 

chucksmith$ find libretro-mesens/ -iname "*dylib*"
libretro-mesens//Libretro/mesen-s_libretro_ios.dylib

[jet082]

I have tested some sample content and it runs on iOS-arm64.

[SourMesen]

Thanks for the pull request! (And sorry for the super long delay on this)