RFR 8047934: Adding new API for unlocking diagnostic argument

[Jiangli Zhou]

Hi,

Please review following change for adding new API that can be used to 
unlock a specific diagnostic argument (JDK-8047934 
<https://bugs.openjdk.java.net/browse/JDK-8047934>):

   http://cr.openjdk.java.net/~jiangli/8047934/webrev.00/

Tested with JPRT and jtreg hotspot/test/runtime/SharedArchiveFile tests.

Thanks,
Jiangli