RFR: Make root iterator functions non-static

[Per Liden]

Hi,

We have an upcoming need for non-static functions in the roots 
iterators. This patch turns all do_* function into non-static member 
functions.

http://cr.openjdk.java.net/~pliden/zgc/roots_iterator_non_static/webrev.0/

Here's also two follow up patches to remove the alignment on 
ZSerialOopsDo/ZParallelOopsDo (which just seems to waste memory) and 
another patch to check the claimed state before trying to CAS in 
ZSerialOopsDo.

http://cr.openjdk.java.net/~pliden/zgc/roots_iterator_remove_cache_alignment/webrev.0/

http://cr.openjdk.java.net/~pliden/zgc/roots_iteratoc_claimed_check_first/webrev.0/

/Per