Use libc++ equivalent of std::__ostream_insert()

Use libc++ equivalent of std::__ostream_insert()

Summary:
- In libstdc++, existing code uses the internal std::__ostream_insert()
method to write a formatted string that can include '\0' characters.
This internal method doesn't exist in libc++. Instead, use the
relevant internal bits.

Test Plan:
- fbconfig -r folly && fbmake runtests
- ./configure && make check on Mac OS X

Reviewed By: njormrod@fb.com

FB internal diff: D1108540