NSZone: implement NSMakeCollectable()

Implement NSMakeCollectable(), an interface which makes CoreFoundation
objects eligible for garbage collection.

Since garbage collection is a totally deprecated API, its implementation
is a no-op, which may cause objects that rely on it to leak. However, it
doesn't look like supporting actual garbage collection at this point
will bring a gain to the project -- and having this function available
will avoid breaking builds that rely on it.

Headers/Foundation/NSZone.h

@@ -320,6 +320,14 @@ NSReallocateCollectable(void *ptr, NSUInteger size, NSUInteger options);
 
 #endif
 
+static inline id NSMakeCollectable(const void *cf) {
+#if __has_feature(objc_arc)
+    return nil;
+#else
+    return (id)cf; // Unimplemented; garbage collection is deprecated.
+#endif
+}
+
 #if	defined(__cplusplus)
 }
 #endif