atomic.h: Fix OSAtomic on PPC64
Cast return of sizeof() to socklen_t

It caused a warning about an implicit cast on OS X/PPC64.
Don't use objc_initializeClassPair on OS X/PPC64

OS X/PPC64 is limited to 10.5 and thus it does not exist there, even
though __OBJC2__ is defined.
PLATFORMS.md: Add Win64 and Win64 forwarding