xen/include: avoid using a compiler extension for BUILD_BUG_ON_ZERO

Redefine BUILD_BUG_ON_ZERO to avoid using a compiler extension
that gives an acceptable semantics to C99 undefined behavior 58
("A structure or union is defined as containing no named members
(6.7.2.1)").

The first definition includes an additional named field of type
char.

The chosen ill-formed construct for the second definition is a struct
with a named bitfield of width 0 when the condition is true,
which prevents the UB without using the compiler extension while keeping
the semantic of the construct.

The choice of the bitwise AND operation to bring the result to 0
when cond is false boils down to possibly better portability.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/include/xen/lib.h

@@ -51,9 +51,10 @@
    e.g. in a structure initializer (or where-ever else comma expressions
    aren't permitted). */
 #define BUILD_BUG_ON_ZERO(cond) \
-    sizeof(struct { _Static_assert(!(cond), "!(" #cond ")"); })
+    (sizeof(struct { char c; _Static_assert(!(cond), "!(" #cond ")"); }) & 0)
 #else
-#define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); })
+#define BUILD_BUG_ON_ZERO(cond) \
+    (sizeof(struct { unsigned u : !(cond); }) & 0)
 #define BUILD_BUG_ON(cond) ((void)BUILD_BUG_ON_ZERO(cond))
 #endif