A reference to the caches array was embedded directly into the builtin
code and this allowed sharing objects between contexts.

Unfortunately, clearing the cache on GC won't prevent sharing so we
either have to have per-context builtin code or load the cache
indirectly from the current context. This change implements the second
approach. The first approach may be interesting to consider in the
future for some perfomance critical functions, and the current
approach can still be improved by putting the caches directly into the
global context (or even global objects).

Review URL: http://codereview.chromium.org/1731002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4486 ce2b1a6d-e550-0410-aec6-3dcde31c8c00