Auto merge of #972 - gnzlbg:packed, r=alexcrichton

Fix undefined-behavior on MacOSX structs in stdbuilds

Some MacOSX structs have an incorrect layout that results in undefined behavior. This is because on `x86_64` the MacOSX kernel headers define these using `#pragma pack 4`.

This PR fixes their layout using `repr(packed(4))` . Since it is only available on nightly, it is only enabled for stdbuilds .
diff --git a/src/unix/notbsd/linux/mod.rs b/src/unix/notbsd/linux/mod.rs
index 956a797..d4a3bb9 100644
--- a/src/unix/notbsd/linux/mod.rs
+++ b/src/unix/notbsd/linux/mod.rs
@@ -1917,6 +1917,11 @@
         fd: ::c_int,
         newfd: ::c_int,
     ) -> ::c_int;
+    pub fn fread_unlocked(ptr: *mut ::c_void,
+        size: ::size_t,
+        nobj: ::size_t,
+        stream: *mut ::FILE
+    ) -> ::size_t;
 }
 
 cfg_if! {