RFR: Bulk backport to shenandoah/jdk11

[Roman Kennke]

As subject says. Most of this is:
- Self-fixing barriers
- Cleanups of shared-code-changes
- Bug-fixes

Notable changeset is:
8217717: ZGC: Broken oop map in C1 load barrier stub

.. which I fast-pathed here because it's needed for C1 sfx barriers. I
also proposed to backport the same changeset to jdk11u.

I also cut out all changes relating to concurrent-class-unloading
because the infrastructure for this is not in jdk11, and also concurrent
roots processing and anything that has to do with native barriers, which
we *could* do, but it would only affect JNI roots. Which doesn't seem
worth the risk and complexity of carrying over all the native barrier stuff.

Changesets:
http://cr.openjdk.java.net/~rkennke/backport-shjdk11-2020-01-13/changesets.txt

Webrev:
http://cr.openjdk.java.net/~rkennke/backport-shjdk11-2020-01-13/webrev.00/

Testing: hotspot_gc_shenandoah (fastdebug, release, x86 - will do
aarch64 right after push)

Ok?

Roman