RFR [11u]: JDK-8212100: AppCDS crashes for some uses with JRuby (back port)

[Jiangli Zhou]

Please review the back port for JDK-8211956.

   webrev: http://cr.openjdk.java.net/~jiangli/8212100/webrev.00/

   parent bug: https://bugs.openjdk.java.net/browse/JDK-8211956

   back port bug: https://bugs.openjdk.java.net/browse/JDK-8212100

The back port was created without merge. One additional assert in 
FileMapInfo::verify_region_checksum was added based on the review 
feedback of the original webrev for JDK 12.

Tested with jruby, tier1-tier5.

Thanks,

Jiangli