"Do not cheat and call your languages' equivalent of str_reverse."
I have to agree with an above commenter here. While I understand the intent of the task you've set, you make it seem like this having to add it is a bad thing. Frankly, my first instinct would be to shoot for that with the idea that "Hey, I know the language, DRY, and surely this is a trick question just to weed out the people that can't focus on solving problems efficiently and quickly."
Considering I could write the function myself without trouble, I'd be more likely, after finding out I'd been dismissed for using the native function, to post about the company as "a company that doesn't even know the languages they are using and obviously using non-programmers to do the hiring."
Basically, if you are going to ask for coding samples or actual demonstrations of code writing, make sure you clarify what you want. After all, while you want a coder, you also want someone who get's the job done efficiently. And punishing someone for solving your problem within the limits outlined quickly and efficiently really suggests that maybe you aren't a great place to work.
Note, I'm not suggesting that this is the OP's work place. Rather, a warning to those who would use these methods.