RFR: 8230878: ZGC: Use thread_local instead of __thread

Wed Sep 11 20:48:03 UTC 2019

We're currently using the gcc specific __thread syntax for thread local 
data. We should make those declarations compiler agnostic.

This patch depends on the proposal to rename THREAD_LOCAL_DECL to 
thread_local (JDK-8230877), which is currently out for review on 
hotspot-dev.

Bug: https://bugs.openjdk.java.net/browse/JDK-8230878
Webrev: http://cr.openjdk.java.net/~pliden/8230878/webrev.0

/Per