compiler_types.h: Attributes: Add __counted_by_ptr macro

Introduce __counted_by_ptr(), which works like __counted_by(), but for
pointer struct members.

struct foo {
	int a, b, c;
	char *buffer __counted_by_ptr(bytes);
	short nr_bars;
	struct bar *bars __counted_by_ptr(nr_bars);
	size_t bytes;
};

Because "counted_by" can only be applied to pointer members in very
recent compiler versions, its application ends up needing to be distinct
from flexibe array "counted_by" annotations, hence a separate macro.

This is a reworking of Kees' previous patch [1].

Link: https://lore.kernel.org/all/20251020220118.1226740-1-kees@kernel.org/ [1]
Co-developed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Bill Wendling <morbo@google.com>
Link: https://patch.msgid.link/20260116005838.2419118-1-morbo@google.com
Signed-off-by: Kees Cook <kees@kernel.org>

include/uapi/linux/stddef.h

@@ -72,6 +72,10 @@
 #define __counted_by_be(m)
 #endif
 
+#ifndef __counted_by_ptr
+#define __counted_by_ptr(m)
+#endif
+
 #ifdef __KERNEL__
 #define __kernel_nonstring	__nonstring
 #else