RFR (tedious) 8230586 Encapsulate fields in filemap.hpp

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8230586
http://cr.openjdk.java.net/~iklam/jdk14/8230586-encapsulate-filemap-hpp.v01/

Many of the member fields in filemap.hpp are declared with public 
accessibility.
This is contrary to HotSpot coding style, and also makes the code
unnecessarily intertwined.

I made the fields private and added accessor functions where necessary.

Also, FileMapInfo::_header should not be accessed directly from other code.
I added accessor functions similar to FileMapInfo::core_spaces_size()
to access information stored in the header.

Testing: hs-tier1, hs-tier2

Thanks
- Ioi