Port string stub for keyed loads to x64 and ARM.

BUG=566
TBR=ager@chromium.org
TEST=test/mjsunit/string-index.js

Original code review: http://codereview.chromium.org/1628003

Review URL: http://codereview.chromium.org/1567024

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4348 ce2b1a6d-e550-0410-aec6-3dcde31c8c00