[aarch64-port-dev ] Shenandoah import (2nd attempt)

[Roman Kennke]

So here's my second attempt at importing Shenandoah into the Aarch64-
Shenandoah integration repository. Compared to the previous one, it
adds some missing barriers that have been found by testing the old one,
and it now comes as a single big patch. Webrev:

http://cr.openjdk.java.net/~rkennke/jdk8u-aarch64-shenandoah/webrev.01/

(If it doesn't show up correctly, it means it hasn't completed
uploading yet, just wait a little).

Please review and commit if you find it ok.

Roman