RFR: 8210321: Create NO_KEEPALIVE CLD holder accessor

[Erik Österlund]

Hi,

In order to perform concurrent class unloading, ZGC needs a way to peek 
the holder without keeping it alive.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8210321/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8210321

Thanks,
/Erik