RFR[S] 8209657 Refactor filemap.hpp to simplify integration with Serviceability Agent

[Ioi Lam]

[Resending to include bug number in e-mail subject line]


https://bugs.openjdk.java.net/browse/JDK-8209657
http://cr.openjdk.java.net/~iklam/jdk12/8209657-shared-FileMapHeader-decl.v01/

Summary:

The CDS FileMapHeader type was big, and was duplicated 4 times in our 
sources.
I moved the parts that's common to HotSpot and Serviceability Agent into 
a new
common header file, cds.h.

I also did various clean up in filemap.cpp/hpp:

- avoid using unwieldy nested types such as 
FileMapInfo::FileMapHeader::space_info
- added convenience function space_at(), so you have

   struct FileMapInfo::FileMapHeader::space_info* si =
         &_header->_space[i];
=>
   CDSFileMapRegion* si = space_at(i);


Testing:

hs tiers 1,2,3 on all supported platforms.