[aarch64-port-dev ] RFR: Backport Shenandoah since last merge

[Roman Kennke]

This is a fairly huge backport from Shenandoah's dev line (then jdk9) to
shenandoah/jdk8u and from there to aarch64-port/jdk8u-shenandoah. It brings:

- bunch of new features (e.g. a separate update-refs phase, parallel and
concurrent code cache scanning, etc)

- much improved performance (both macro- and micro-performance)

- much improved stability (we did test this fairly extensively, and
added many regression tests)

- much improved usability (e.g. better option handling, much more useful
output)

- many bugfixes and tests and code improvements

- lesser diff compared to upstream

All of this has baked considerably in shenandoah/jdk8u for a while, gone
through testing and testing and testing (also, testing by interested
early adopters) and fixing. I simply transplanted all relevant
changesets from shenandoah/jdk8u to aarch64-port/jdk8u-shenandoah with
no fuzz or any problems.

It's then been tested again by running the hotspot_gc_shenandoah jtreg
test group (i.e. the whole Shenandoah jtreg test suite), again without
issues.

A list of all changesets can be found here:

http://cr.openjdk.java.net/~rkennke/aarch64-import-2017-07-24/out.txt
<http://cr.openjdk.java.net/%7Erkennke/aarch64-import-2017-07-24/out.txt>

The full webrev:

http://cr.openjdk.java.net/~rkennke/aarch64-import-2017-07-24/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/aarch64-import-2017-07-24/webrev.00/>


Ok to push?