Previously, it always used stderr. Slight shuffle of the first line
of the crash due to reusing code from rb_bug():

```diff
-Assertion Failed: /ruby/object.c:649:rb_obj_itself:false
+/ruby/object.c:649: Assertion Failed: rb_obj_itself:false
```

Tested locally to confirm that it writes to the file given with
RUBY_CRASH_REPORT. Follow-up for [Feature #19790].