util: Avoid buggy std::filesystem:::create_directories() call

Summary:
Compiled with some libstdc++ versions (e.g., on Ubuntu 20.04, Debian 11) std::filesystem:::create_directories() call fails to handle symbol links
properly.

This is a backport of core#24266

Note that this fixes also a redundant "/" being added to path when BaseParams().DataDir().empty(), so this is a dependency for D13730

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13735