RFR: Fix Shenandoah non-PCH build

[Roman Kennke]

Am 20.08.2018 um 14:57 schrieb Zhengyu Gu:
> Non-PCH build broke again.
> 
> http://cr.openjdk.java.net/~zgu/shenandoah/sh-non-pch/webrev.00/index.html
> 
> Test:
>   non-PCH builds (fastdebug and release)
> 
> 
> Thanks,

Yes, please push! Thanks!
Roman