Clean up `_ctypes`/`libffi` build, particularly on macOS

[zware]

bpo-28491/gh-72677 was closed after bpo-41100/gh-85272, but Modules/_ctypes/libffi_osx wasn't actually removed, just made inaccessible to any build. In the course of figuring out whether it is in fact unused now, I also stumbled across Modules/_ctypes/darwin, which also appears to be a dlfcn.h shim for Mac OS X 10.2 and earlier, and as far as I can tell has been unused for years. And, of course, with no local copy of libffi on any platform anymore, there's no need for --with-system-ffi to stick around, which also pointed me towards a needless -DMACOSX in the macOS build of _ctypes. I'll be attaching a series of PRs for 3.12 (only) removing all of these, each of which can be done independently.

Linked PRs

• gh-100540: Remove obsolete Modules/_ctypes/darwin/ dlfcn shim #100541
• gh-100540: Remove unnecessary '-DMACOSX' for ctypes on macOS #100542
• gh-100540: Remove unused Modules/_ctypes/libffi_osx/ #100543
• gh-100540: Remove obsolete '--with-system-ffi' configure option #100544