FStream: Add MinGW support

std::basic_filebuf::open(const wchar_t *) isn't part of C++ standard
and it's only present for MSVC but it isn't present in libstdc++ (MinGW)
so we implement this functionality using GNU libstdc++ stdio_filebuf
extension and _wfopen function.

Change-Id: I7df7a1b22ba272db23c8d962cb66ec71a278ef06