[libcxx][tests] Introduce 32-bit feature and use it for stringstream gcount test

This will avoid hardcoding all unsupported targets, since even after one
more follow up fix [1], there is one more failure.

  [1]: https://reviews.llvm.org/D150886

Plus, if you want to run it locally on some target that CI does not
covers, it could also false-positively fail, which is not good.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D151046