RFR: 8210063: ZGC: Enable load barriers for IN_NATIVE runtime barriers

[Erik Österlund]

Hi,

In an effort to scan more (strong) root sets concurrently, we need to 
support IN_NATIVE oops not being fixed up in safepoints. Both the 
ZBarrierSetAssembler, ZBarrierSetC1 and ZBarrierSetC2 already have 
IN_NATIVE load barriers on for all strong oops. However, ZBarrierSet 
does not yet enable it. I propose to enable it so we can scan more 
things concurrently.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8210063/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8210063

Thanks,
/Erik