Remove G_OS_DARWIN in favour of platform-specific defines

Following on from !2997 (merged), @smcv suggested:

I actually wonder whether we should also remove the new G_OS_DARWIN from the 2.75.x branch, and replace uses of it with __APPLE__, reverting most of !2981 (merged). Strictly speaking __APPLE__ could either be Mac OS Classic (ye olde cooperatively-multitasking, non-privilege-separated OS with its various non-Unix oddities) or the Darwin family (iOS, tvOS and modern macOS), but GLib no longer supports Mac OS Classic anyway, so anyone who is compiling GLib can assume that __APPLE__ means the Darwin family.

That seems consistent with the approach we went with in !2997 (merged), and less likely to lead to confusion about why G_OS_DARWIN exists but G_OS_FREEBSD doesn’t.

We’d need to decide on this before the 2.75 API freeze, as G_OS_DARWIN has not yet been in a stable release.