Backport minimal compiler_attributes.h to support GCC 9

This adds support for __copy to v4.9.y so that we can use it in
init/exit_module to avoid -Werror=missing-attributes errors on GCC 9.

Link: https://lore.kernel.org/lkml/259986242.BvXPX32bHu@devpool35/


Cc: <stable@vger.kernel.org>
Suggested-by: Rolf Eike Beer <eb@emlix.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>