X87: Simplify string copy in SubStringStub.

port r21756.

original commit message:
  The optimizations are unnecessary since the maximum
  string length they operate on is currently 12.

R=yangguo@chromium.org, bmeurer@chromium.org
  BUG=352155
  LOG=N

Review URL: https://codereview.chromium.org/326353002

Patch from Chunyang Dai <chunyang.dai@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21772 ce2b1a6d-e550-0410-aec6-3dcde31c8c00