RFR(S): JDK-8032250 : Add trace event for VM flag changes

[Staffan Larsen]

This is a patch to capture a trace event whenever a manageable VM flag is 
changed in runtime.

webrev: http://cr.openjdk.java.net/~sla/8032250/webrev.00/
bug: https://bugs.openjdk.java.net/browse/JDK-8032250

Thanks,
/Staffan