Must the mrb_sys_fail() function raise an exception

Normal return of control from the `mrb_sys_fail()` function has unexpected results.
Example: double free, malloc with huge size used as error integer

The following is an example of an actual crash:

```console
% cat crash.rb
def SystemCallError._sys_fail(*a)
  nil
end

File.readlink("/404")

% bin/mruby crash.rb
zsh: segmentation fault (core dumped)  bin/mruby crash.rb
```

include/mruby/error.h

@@ -23,7 +23,7 @@ struct RException {
 #define mrb_exc_ptr(v) ((struct RException*)mrb_ptr(v))
 #define MRB_EXC_MESG_STRING_FLAG 0x100
 
-MRB_API void mrb_sys_fail(mrb_state *mrb, const char *mesg);
+MRB_API mrb_noreturn void mrb_sys_fail(mrb_state *mrb, const char *mesg);
 MRB_API mrb_value mrb_exc_new_str(mrb_state *mrb, struct RClass* c, mrb_value str);
 #define mrb_exc_new_lit(mrb, c, lit) mrb_exc_new_str(mrb, c, mrb_str_new_lit(mrb, lit))
 MRB_API mrb_value mrb_make_exception(mrb_state *mrb, mrb_int argc, const mrb_value *argv);