RFR: 8196197 - Enable the make system to calculate concurrency for JDK tests

[Christian Tornqvist]

Please review this small change that makes the JDK make test target use the same method for calculating jtreg concurrency as Hotspot. Verified locally and in Mach5.

Webrev: http://cr.openjdk.java.net/~ctornqvi/webrev/8196197/webrev.00/ <http://cr.openjdk.java.net/~ctornqvi/webrev/8196197/webrev.00/>

Thanks,
Christian