Resolve a ctest FIXME regarding use of size_of in array lengths

(backport <rust-lang#4713>)
(cherry picked from commit 6f0ebb8)
[ note that there was no FIXME comment on 0.2 - Trevor ]

Author: tgross35

src/unix/linux_like/linux/musl/mod.rs

@@ -136,10 +136,7 @@ s! {
         pub aio_offset: off_t,
         __next: *mut c_void,
         __prev: *mut c_void,
-        #[cfg(target_pointer_width = "32")]
-        __dummy4: [c_char; 24],
-        #[cfg(target_pointer_width = "64")]
-        __dummy4: [c_char; 16],
+        __dummy4: [c_char; 32 - 2 * size_of::<*const ()>()],
     }
 
     #[repr(align(8))]