Clean up _ctypes/libffi build, particularly on macOS #100540
Description
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.