[libcxx] Replace func_name with __name__ for compatibility with Python 3

Summary:
The __name__ attribute is the correct way to get a function name in
Python 3. This also works with Python 2.

Reviewers: jroelofs, EricWF

Subscribers: christof, ldionne, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D71136