RFR: [10] Backports for fixing Shenandoah JDK10 non-PCH build

[Zhengyu Gu]

A couple of backports to fix Shenandoah JDk10 build without precompiled 
headers.

Both patches were applied cleanly.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/sh-jdk10-non-PCH/webrev.00/

Test:

   hotspot_gc_shenandoah (fastdebug + release)

Thanks,

-Zhengyu