[libcxx] r205138 - ARM64: use the alternate string layout on Apple platforms.

[Tim Northover]

> In this and the other commit to libc++, I think it is better to more
> explicitly phrase the check for arm64 as
>
> #if __APPLE__ && (defined(__arm64__) || defined(__aarch64__))

I didn't write the code, but I'm assuming the reasoning was that we'd
want the improved version in any future ABIs we create. "OK, we need
to grudgingly use the old one on i386 etc, but not in future".

> Also, it is better to use defined(__APPLE__) to be consistent with the
> rest of the file (and avoid warnings about using macros that are not
> defined?)

That's a good idea though. r205150

Cheers.

Tim.