RFR: JDK-8226695: Shenandoah: Wire C1 and C2 IN_NATIVE barrier

[Roman Kennke]

IN_NATIVE accesses in compiled code should go to runtime, in order to be
able to check reachability of accessed objects, and possibly offer NULL
for unreachable objects.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8226695
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8226695/webrev.00/

Testing: hotspot_gc_dev

Ok?

Roman