[libcxx] Fix crash in std::stringstream with payload >= INT_MAX

stringstream does works for payload > INT_MAX, however
stringstream::gcount() can break the internal field (__nout_) and this
breaks the stringstream itself, and so the program will crash.

Fix this, by using __pbump(streamsize) over pbump(int)

Note, libstdc++ does not have this bug.

Reviewed By: #libc, ldionne, Mordante

Spies: arichardson, Mordante, philnik, ldionne, libcxx-commits, mikhail.ramalho

Differential Revision: https://reviews.llvm.org/D146294