I couldn't find this documented anywhere and it took me a while to figure out so I'm posting here in case anyone finds it useful.
When using Link All Assemblies if you want to tell the linker to skip entire assemblies, you can do it by putting this in the "Additional mtouch arguments" of your top-level assembly
If you want the linker to skip multiple assemblies, you include multiple linkskip arguments:
If you try to use a list of assemblies (for example,
--linkskip=FirstAssembly;SecondAssembly), the linker will silently fail to skip anything. I swear that this worked in an older version of Xamarin.iOS, but it doesn't work today (Xamarin.iOS 7.2.6).