Skeleton code for mapping code cache from CDS

[ioi.lam at oracle.com]

In follow up to today's Leyden meeting:

Here's the skeleton code in the premain branch that allows you to

- allocate space from the "cached code" section of the CDS archive

- Store pointers to (a) metadata and (b) other objects that are 
allocated within "cached code" section. These pointers will be 
automatically relocated during the production run.

https://github.com/openjdk/leyden/blob/21640b10188c8bfb414505043110d6da15080d16/src/hotspot/share/code/SCCache.cpp#L429-L504


(Note: currently, the AOT-compiled methods are NOT stored in this 
section, but are stored in a separate file).