[libc++] Work around dynamic linking of stringstream::str() on Windows

Also `istringstream::str()` and `ostringstrem::str()`.

https://github.com/llvm/llvm-project/issues/40363 causes `str()`
to be dllimport'ed despite _LIBCPP_HIDE_FROM_ABI.
This is a temporary solution until #40363 is fixed.

Reviewed By: #libc, thakis, philnik

Differential Revision: https://reviews.llvm.org/D157602