uapi: stddef.h: Fix __DECLARE_FLEX_ARRAY for C++

__DECLARE_FLEX_ARRAY(T, member) macro expands to

	struct {
		struct {} __empty_member;
		T member[];
	};

which is subtly wrong in C++ because sizeof(struct{}) is 1 not 0,
changing UAPI structures layouts.

This can be fixed by expanding to

	T member[];

Now g++ doesn't like "T member[]" either, throwing errors on
the following code:

	struct S {
		union {
			T1 member1[];
			T2 member2[];
		};
	};

or

	struct S {
		T member[];
	};

Use "T member[0];" which seems to work and does the right thing wrt
structure layout.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Fixes: 3080ea5 ("stddef: Introduce DECLARE_FLEX_ARRAY() helper")
Link: https://lore.kernel.org/r/97242381-f1ec-4a4a-9472-1a464f575657@p183
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: Alexey Dobriyan

include/uapi/linux/stddef.h

@@ -29,6 +29,11 @@
 		struct TAG { MEMBERS } ATTRS NAME; \
 	}
 
+#ifdef __cplusplus
+/* sizeof(struct{}) is 1 in C++, not 0, can't use C version of the macro. */
+#define __DECLARE_FLEX_ARRAY(T, member)	\
+	T member[0]
+#else
 /**
  * __DECLARE_FLEX_ARRAY() - Declare a flexible array usable in a union
  *
@@ -44,6 +49,7 @@
 		struct { } __empty_ ## NAME; \
 		TYPE NAME[]; \
 	}
+#endif
 
 #ifndef __counted_by
 #define __counted_by(m)