RFR: Pick up jdk-15+15

[Roman Kennke]

I'd like to put a change (JDK-8241605) in shenandoah/jdk to get some
testing before I push it to jdk/jdk. In preparation for this, I'd like
to pick up jdk-15+15 into shenandoah/jdk. The list of changesets:

http://cr.openjdk.java.net/~rkennke/upstream-merge-jdk-15%2b15/changesets.txt

Merge was trivial without any conflicts.

Testing: hotspot_gc_shenandoah

Ok?

Roman