Backport minimal compiler_attributes.h to support GCC 9

BugLink: https://bugs.launchpad.net/bugs/1845038

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

include/linux/compiler.h

@@ -52,6 +52,22 @@ extern void __chk_io_ptr(const volatile void __iomem *);
 
 #ifdef __KERNEL__
 
+/*
+ * Minimal backport of compiler_attributes.h to add 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.
+ */
+#ifndef __has_attribute
+# define __has_attribute(x) __GCC4_has_attribute_##x
+# define __GCC4_has_attribute___copy__                0
+#endif
+
+#if __has_attribute(__copy__)
+# define __copy(symbol)                 __attribute__((__copy__(symbol)))
+#else
+# define __copy(symbol)
+#endif
+
 #ifdef __GNUC__
 #include <linux/compiler-gcc.h>
 #endif