[libcxx] [Windows] Make a more proper implementation of strftime_l for mingw with msvcrt.dll

This also makes this function consistent with the rest of the
libc++ provided fallbacks.

The locale support in msvcrt.dll is very limited anyway; it can
only be configured processwide, not per thread, and it only seems
to support the locales "C" and "" (the user set locale), so it's
hard to make any meaningful automatic test for it. But manually tested,
this change does make time formatting locale code in libc++ output
times in the user requested format, when using locale "".

Differential Revision: https://reviews.llvm.org/D69554