CYGWIN: warning on missing secure randomness function

[carlo-bramini]

Bug report

Bug description:

While compiling latest sources of Python on CYGWIN, this warning is printed:

unable to use secure randomness

This happens because the platform is not handled into Objects\mimalloc\prim\unix\prim.c.
However, this can be fixed very easily because CYGWIN provides arc4random_buf(), which is already used by the ports for other plaforms.
So, the issue can be fixed by simply adding an || defined(__CYGWIN__) near __FreeBSD__, __NetBSD__ and others.

This is a patch for mimalloc, so see also #113141 for reference.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-148461: use arc4random_buf() on CYGWIN #148462