[libc++] Fix ssize test that made an assumption about ptrdiff_t being 'long'

On some platforms like armv7m, the size() method of containers returns
unsigned long, while ptrdiff_t is just int. Hence, std::ssize_t ends up
being long, which is not the same as ptrdiff_t. This is usually not an
issue because std::ptrdiff_t is long, so everything works out, but it
breaks on some more exotic architectures.

Differential Revision: https://reviews.llvm.org/D114563