MIPS: Add initial hydrogenized NewStringAddStub.

MIPS: Add initial hydrogenized NewStringAddStub.

Port r17635 (14180b1)

Original commit message:
The new stub is enabled via the --new-string-add flag, which is
disabled by default. For now, it's only a stripped down version
of the native StringAddStub, it's still work-in-progress.

BUG=v8:2990
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/69953005

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

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17664 ce2b1a6d-e550-0410-aec6-3dcde31c8c00