Call unicode API on Windows

Summary:
Rationale from issue core#13869:

Currently Bitcoin uses the ANSI encoding on Windows so that it would affected by user's language settings, they might not able to open some non-ASCII name files.

This should unbreak the windows build, which was broken by D10801.

This is a backport of core#13888

Depends on D10816

Test Plan: @bot build-win64 gitian-win

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10815