RFR: 8166790: Add stress test GCBasher

[Erik Helin]

Hi all,

this patch adds a new GC stress test called GCBasher. GCBasher builds up
large (well, for some definiton of large) object graphs by figuring out
the relations between classes in the JDK. The test usually stresses the
GC quite a lot, especially when run with a smaller heap.

The changes in the top-level repository are for JPRT. JPRT will now run
the jtreg test GCBasher instead of the old version.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8166790

Webrev:
- hotspot: http://cr.openjdk.java.net/~ehelin/8166790/hotspot/00/
- top: http://cr.openjdk.java.net/~ehelin/8166790/top/00/

Testing:
- Running the new jtreg test locally on Linux x86-64:
  $ jtreg -jdk:build/fastdebug/jdk hotspot/test/gc/stress/TestGCBasher.java

  (can also be run via: $ make test TEST=hotspot_fast_gc_gcbasher)
- JPRT

Thanks,
Erik