RFR: JDK-8131645 [ARM64] crash on Cavium when using G1

[Eric Caspole]

Hi everybody,
Could you review this fix?

https://bugs.openjdk.java.net/browse/JDK-8131645

Webrev:
http://cr.openjdk.java.net/~ecaspole/JDK-8131645/00/webrev/

The problem was that without a store membar on the CodeRootSetTable 
creation, readers could see invalid memory leading to an assert or 
crash. This is more noticeable on ARM64 with relaxed memory semantics.
Thanks Stefan K for noticing the exact spot for this fix.

This ran all night with the failing test with no crashes and passes JPRT.
Thanks,
Eric