RFR 8213751: ClassLoaderDataGraph::cld_do() should sometimes require CLDG_lock

[coleen.phillimore at oracle.com]

Summary: Give GC own version of cld_oops_do and use cld_do for runtime 
calls.

This also adds a NoSafepointVerifier in the CLDG iterator.

Tested with hs-tier1-3, vmTestbase/nsk, gc, runtime and JFR tests locally.

open webrev at http://cr.openjdk.java.net/~coleenp/8213751.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8213751

Thanks,
Coleen