ARM: Improve SeqStringSetChar implementation.

TEST=/test/mjsunit/lithium/SeqStringSetChar.js
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/23890007

Patch from Rodolph Perfetta <rodolph.perfetta@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16707 ce2b1a6d-e550-0410-aec6-3dcde31c8c00