8212205 (XS) VM asserts after CDS archive has been unmapped

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8212205
http://cr.openjdk.java.net/~iklam/jdk12/8212205-crash-after-cds-unmap.v01/

This is a silly bug but it happens only rarely, when address space 
randomization
causes insufficient memory to be reservable at addresses required by 
CDS. In this
case, the CDS archive is unmapped, but we forget to clean up
_shared_metaspace_{base,top}. As a result, the JVM will misidentify 
metaspace
objects allocated in certain address ranges to be "shared", in the 
following function:

class MetaspaceObj {
   bool is_shared() const {
     return (((void*)this) < _shared_metaspace_top && ((void*)this) >= 
_shared_metaspace_base);
   }
}

The fix is to reset those pointers after CDS has been unmapped.

Thanks
- Ioi