RFR: Bulk integration from shenandoah/jdk8

[Roman Kennke]

Hello all,

I'd like to propose integration of outstanding changes and backports
from shenandoah/jdk8.

This time it is rather big. Most significantly, it includes LRB,
elimination of forwarding pointer, the new x86_32 port, the Traversal
GC, plus a whole lot of related bugfixes.

The list of changes:
http://cr.openjdk.java.net/~rkennke/aarch64-shenandoah-integration-2019-12-09/changesets.txt

Shared-only webrev:
http://cr.openjdk.java.net/~rkennke/aarch64-shenandoah-integration-2019-12-09/webrev-shared-only.00/

Full webrev:
http://cr.openjdk.java.net/~rkennke/aarch64-shenandoah-integration-2019-12-09/webrev-all.00/

There are quite many shared-code changes involved in this. However, most
of them actually revert/remove existing shared-code changes vs upstream
jdk8u, so the net effect is a drastic improvement. We added a few new
hooks (for the LRB), but we kept it inside the following pattern
wherever possible:

#ifdef INCLUDE_ALL_GCS
if (UseShenandoahGC) {...
}
#endif

Also, this change includes some renaming and moving-around of
(Shenandoah) files to keep Shenandoah code in Shenandoah subdirectories,
and follow the naming scheme of >=11 as much as possible.

Can I please get a review of this?

Thanks,
Roman