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 .