New experimental GC stress mode

Tries to do a GC for every unique call stack (up to 16 frames).
The goal is to catch moving GC bugs and lock violations without being
rediculously slow. Some tests fail on 64 bits, 32 bit host doesn't
work. N5 is booting.

Added runtime -Xgc options: gcstress and nogcstress.

Bug: 21664466

Change-Id: Icb8e420f2048e8ee83bcca7937563166a2638f5c