RFR (XS) 8064701: Some CDS optimizations should be disabled if bootclasspath is modified by JVMTI

[Ioi Lam]

Please review a very small fix:

http://cr.openjdk.java.net/~iklam/8064701-append-boot-v1/hotspot/

Bug: Some CDS optimizations should be disabled if bootclasspath is 
modified by JVMTI
     https://bugs.openjdk.java.net/browse/JDK-8064701


Summary of fix:

     This change adds an API so that the class loader is notified when 
JVMTI modifies
     the boot classpath. Further CDS optimizations can use this API to 
disable
     optimizations that may be invalidated by boot classpath modifications.

Also added white box testing API for invoking JVMTI boot/system classpath
     modifications for further CDS testing needs.

Tests:

     JPRT

Thanks
- Ioi