RFR(T) 8240267 VM fails to start with CDS enabled but JVMTI disabled

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8240267
http://cr.openjdk.java.net/~iklam/jdk15/8240267-cds-enabled-jvmti-disabled.v01/

This affects non-product builds only. The fix is to put the 
JVMTI-related asserts
inside JVMTI_ONLY().

Thanks
- Ioi