RFR: Simplify JNI critical support

[Aleksey Shipilev]

On 09/21/2016 01:10 PM, Roman Kennke wrote:
> http://cr.openjdk.java.net/~rkennke/jni_critical/webrev.00/

Impressive.

Minor comments:

 *) The test/gc/shenandoah are better be "hotspot_gc_shenandoah". We
will select "fast" subset later, as our test count grows.

 *) I am fuzzy on the synchronization details here, but shouldn't
_critical_pins be volatile and/or read through Atomic::<something> as well?

Thanks,
-Aleksey