io: free Windows operation data with _aligned_free()

`_dispatch_operation_perform()` uses `_aligned_malloc()` on Windows to
allocate the data buffer. This means that we cannot use
`DISPATCH_DATA_DESTRUCTOR_FREE` when we create the `dispatch_data_t`.
Provide a custom destructor which correctly frees the buffer.

Discovered while porting the dispatch_io test to Windows.

Author: adierking

src/io.c

@@ -2483,8 +2483,14 @@ _dispatch_operation_deliver_data(dispatch_operation_t op,
 	if (op->direction == DOP_DIR_READ) {
 		if (op->buf_len) {
 			void *buf = op->buf;
+#if defined(_WIN32)
+			// buf is allocated with _aligned_malloc()
+			data = dispatch_data_create(buf, op->buf_len, NULL,
+					^{ _aligned_free(buf); });
+#else
 			data = dispatch_data_create(buf, op->buf_len, NULL,
 					DISPATCH_DATA_DESTRUCTOR_FREE);
+#endif
 			op->buf = NULL;
 			op->buf_len = 0;
 			dispatch_data_t d = dispatch_data_create_concat(op->data, data);