macro: use __builtin_unreachable on NDEBUG

note that this slightly changes the semantic of assert when NDEBUG is
defined. if there's an extern function call (without attribute pure or
similar) then the compiler has to assume it has side effects and still
emit the function call.

whereas the old assert guaranteed that nothing will be evaluated on
NDEBUG.

Closes: https://github.com/systemd/systemd/issues/29408

src/basic/macro.h

@@ -189,7 +189,7 @@ static inline int __coverity_check_and_return__(int condition) {
 /* We override the glibc assert() here. */
 #undef assert
 #ifdef NDEBUG
-#define assert(expr) do {} while (false)
+#define assert(expr) ({ if (!(expr)) __builtin_unreachable(); })
 #else
 #define assert(expr) assert_message_se(expr, #expr)
 #endif