MIPS: Turn FastNewContextStub into a HydrogenCodeStub.

Port r18764 (c40563f)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/145013010

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18784 ce2b1a6d-e550-0410-aec6-3dcde31c8c00