Add initial hydrogenized NewStringAddStub.

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=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/61893009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17635 ce2b1a6d-e550-0410-aec6-3dcde31c8c00