Use __APPLE__ instead of APPLE_IOS for apple/arm specific features

The "features" enabled by this define are mandatory - whenever building with apple tools, these codepaths must be used, thus using a compiler built-in define makes the code more robust than using a define manually enabled in a project file.

(This is contrary to e.g. renaming the main function to DecMain/EncMain, which only is necessary when building as part of the iOS example projects - you could just as well build the standalone h264dec/h264enc executables as such without renaming the functions.)