8178095: Add GC stress test TestSystemGC

[Erik Helin]

Hi all,

this patch adds a small stress test called TestSystemGC. The test
stresses a full GC implementation by allocating objects of different
life times while concurrently calling System.gc() repeatedly.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8178095

Patch:
http://cr.openjdk.java.net/~ehelin/8178095/00/

Testing:
- make run-test TEST=hotspot/test/gc/stress/systemgc

Thanks,
Erik