Require an isolate parameter for most external reference creation to

Require an isolate parameter for most external reference creation to
avoid TLS access in connection with external references.

Make the isolate accessible via the assembler.

Only for ia32 at this point. If this looks OK to you I will port it.

R=vitalyr@chromium.org

Review URL: http://codereview.chromium.org/6713074

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7305 ce2b1a6d-e550-0410-aec6-3dcde31c8c00