auto merge of #11472 : kud1ing/rust/iOS, r=alexcrichton

This fixes #11336

I guess the type sizes are correct for both OS X and iOS, but i am not certain.
In any case, i'd rather have any iOS build at all, so that we have something to improve upon.

src/libstd/libc.rs

@@ -1160,6 +1160,7 @@ pub struct ip6_mreq {
             }
         }
 
+        #[cfg(target_arch = "arm")]
         #[cfg(target_arch = "x86")]
         pub mod arch {
             pub mod c95 {