MIPS: Reland v8:18458 "Load the global proxy from the context of the target function."

Port r18462 (7024b6d)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/126043002

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18475 ce2b1a6d-e550-0410-aec6-3dcde31c8c00