RFR: 8230878: ZGC: Use thread_local instead of __thread

[Kim Barrett]

> On Sep 11, 2019, at 4:48 PM, Per Liden <per.liden at oracle.com> wrote:
> 
> 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

Looks good.