allow utimes on mac os, only lutimes isn't supported

Fix #6483
Fix https://github.com/boot2docker/boot2docker/issues/398

This will work most of the time, but will still fail on links.
Should we skip the error in `docker cp` for the `LUtimesNano` ?
@vieux When would err be `system.ErrNotSupportedPlatform` for `UtimeNano` ?