[aarch64-port-dev ] RFR: Shenandoah import

[Roman Kennke]

This imports Shenandoah changes from shenandoah/jdk8u/hotspot into the 
integration repository aarch64-port/jdk8u-shenandoah/hotspot .

It includes (and thus supercedes) changes proposed here:

http://mail.openjdk.java.net/pipermail/aarch64-port-dev/2017-August/004854.html

Instead of doing bulk-import I went back to transplanting 
single-changeset import. I can avoid the merge conflicts simply by not 
merging back from aarch64-port/jdk8u-shenandoah but instead from 
aarch64-port/jdk8u. Plus we get the benefit of preserving the original 
changesets, make bisecting more useful, etc.

It also includes newer backports from Aug 21 - today.

Everything has been extensively tested and baked in Shenandoah land.

Notably, it includes a bunch of changes done by Roland that better 
isolates Shenandoah specific code paths in C2 (thus minimizing 
Shenandoah exposure in non-Shenandoah runs).

Besides that, all of it is bugfixing plus performance and usability 
improvements.

The complete list of changesets is here:

http://cr.openjdk.java.net/~rkennke/jdk8u-shenandoah-integration-2017-10-02/changesets.txt 
<http://cr.openjdk.java.net/%7Erkennke/jdk8u-shenandoah-integration-2017-10-02/changesets.txt>

The total webrev is here:

http://cr.openjdk.java.net/~rkennke/jdk8u-shenandoah-integration-2017-10-02/webrev.00/ 
<http://cr.openjdk.java.net/%7Erkennke/jdk8u-shenandoah-integration-2017-10-02/webrev.00/>

Please review! Thanks, Roman