[libcxx] Enable filesystem by default for mingw targets

This feature can be built successfully for windows now. However,
the helper functions for __int128_t aren't available in MSVC
configurations, so don't enable it by default there yet. (See
https://reviews.llvm.org/D91139 for discussion on how to proceed
with things in MSVC environments.)

Differential Revision: https://reviews.llvm.org/D97075