ios12 fix delivery to boehm-gc upstream

The fix is confirmed by users.
Same fixes were merged into boehm-gc code base and delivered as PR232, also issue #231 was created to track case.
Also up-to-date boehm-gc with these changes for RoboVM is available in my branch dkimitsa/boehm-gc. Will investigate if there is any benefit to upgrade to recent boehm-gc once ios12 is released.