Fix shim_arg_size test for 32-bit machines

Author: samrat

tests/compile-fail/shim_arg_size.rs

@@ -5,7 +5,13 @@ extern crate libc;
 // error-pattern: scalar size mismatch
 fn main() {
     extern "C" {
+        // Use the wrong type(ie. not the pointer width) for the `size`
+        // argument.
+        #[cfg(target_pointer_width="64")]
         fn malloc(size: u32) -> *mut std::ffi::c_void;
+
+        #[cfg(target_pointer_width="32")]
+        fn malloc(size: u16) -> *mut std::ffi::c_void;
     }
 
     unsafe {