Mainline PR for Soft/Weak Reference Support

[ioi.lam at oracle.com]

Folks,

I am upstreaming the Soft/Weak Reference Support from leyden to mainline

https://github.com/openjdk/jdk/pull/24757

The implementation of References in the JDK is complex. In this PR, we 
added a new "prepareForAOTCache()" pattern to simplify the kind of 
References that can be cached.

Please see the PR for detail. Any comments/suggestions about this new 
pattern will be greatly appreciated.

- Ioi