lib/attr.h: Use C23-style attributes

They're stricter.  The GNU attributes are too lazy, and can be misused
more easily.  Also, mixing both has its own problems.

Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108796>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

Author: alejandro-colomar

lib/attr.h

@@ -6,11 +6,11 @@
 
 
 #if defined(__GNUC__)
-# define MAYBE_UNUSED                __attribute__((unused))
-# define NORETURN                    __attribute__((__noreturn__))
-# define format_attr(type, fmt, va)  __attribute__((format(type, fmt, va)))
-# define ATTR_ACCESS(...)            __attribute__((access(__VA_ARGS__)))
-# define ATTR_ALLOC_SIZE(...)        __attribute__((alloc_size(__VA_ARGS__)))
+# define MAYBE_UNUSED                [[gnu::unused]]
+# define NORETURN                    [[gnu::__noreturn__]]
+# define format_attr(type, fmt, va)  [[gnu::format(type, fmt, va)]]
+# define ATTR_ACCESS(...)            [[gnu::access(__VA_ARGS__)]]
+# define ATTR_ALLOC_SIZE(...)        [[gnu::alloc_size(__VA_ARGS__)]]
 #else
 # define MAYBE_UNUSED
 # define NORETURN