SI-7237 Always choose ForkJoinTaskSupport

ForkJoinTaskSupport works on Hotspot, Avian and J9,
while ThreadPoolTaskSupport causes the test
test/files/scalacheck/parallel-collections
to reliably hang on all three.
Merge pull request #2348 from paulp/pr/2324-edit

SI-7237 Always choose ForkJoinTaskSupport