Towards compiling with pre-C++11 compilers and nullptr fallback

https://github.com/behdad/harfbuzz/issues/585

src/hb-atomic-private.hh

@@ -89,9 +89,9 @@ typedef int32_t hb_atomic_int_impl_t;
 #define hb_atomic_ptr_impl_cmpexch(P,O,N)	OSAtomicCompareAndSwapPtrBarrier ((void *) (O), (void *) (N), (void **) (P))
 #else
 #if __ppc64__ || __x86_64__ || __aarch64__
-#define hb_atomic_ptr_impl_cmpexch(P,O,N)	OSAtomicCompareAndSwap64Barrier ((int64_t) (O), (int64_t) (N), (int64_t*) (P))
+#define hb_atomic_ptr_impl_cmpexch(P,O,N)	OSAtomicCompareAndSwap64Barrier ((int64_t) (void *) (O), (int64_t) (void *) (N), (int64_t*) (P))
 #else
-#define hb_atomic_ptr_impl_cmpexch(P,O,N)	OSAtomicCompareAndSwap32Barrier ((int32_t) (O), (int32_t) (N), (int32_t*) (P))
+#define hb_atomic_ptr_impl_cmpexch(P,O,N)	OSAtomicCompareAndSwap32Barrier ((int32_t) (void *) (O), (int32_t) (void *) (N), (int32_t*) (P))
 #endif
 #endif