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