RFR(s) 8246382: assert in MetaspaceShared::map_archives

[Calvin Cheung]

bug: https://bugs.openjdk.java.net/browse/JDK-8246382
webrev: http://cr.openjdk.java.net/~ccheung/jdk15/8246382/webrev.00/

The bug is due to the base CDS archive used for dynamic dumping does not 
match the one used during runtime.

Currently, the checking of the base archive header CRC is done in 
DynamicArchive::validate but it is after the assert in 
MetaSpace::map_archives. A fix is to do the check earlier in 
FileMapInfo::validate_header.

Testing: tier1, 2 (in progress)
               all appcds tests passed locally on linux.

thanks,

Calvin