kernel/util: Make exit() available in kernel mode

Since we are using libraries originally intendent for user mode in kernel
mode providing them with some userland functions is inevitable. This
particular patch is to make zlib happy and able to call exit() when
its debug assertions fails.

src/system/kernel/util/kernel_cpp.cpp

@@ -251,6 +251,15 @@ abort()
 	panic("abort() called!");
 }
 
+
+extern "C"
+void
+exit(int status)
+{
+	panic("exit() called with status code = %d!", status);
+}
+
+
 extern "C"
 void
 debugger(const char *message)