use compiler extension instead of C11 noreturn

C11 noreturn isn't available in C++.

new.cc

@@ -1,8 +1,6 @@
 #include <bits/functexcept.h>
 #include <new>
 
-#define noreturn
-
 #include "h_malloc.h"
 #include "util.h"
 

util.h

@@ -2,7 +2,9 @@
 #define UTIL_H
 
 #include <stdint.h>
-#include <stdnoreturn.h>
+
+// C11 noreturn doesn't work in C++
+#define noreturn __attribute__((noreturn))
 
 #define likely(x) __builtin_expect(!!(x), 1)
 #define unlikely(x) __builtin_expect(!!(x), 0)