We were converting an int to a Smi, calling ToString to create a String
object, then appending this String to an IncrementalStringBuilder.
It's much easier and more efficient to just sprintf to a local buffer
and append that instead.

R=titzer@chromium.org, ahaas@chromium.org
BUG=v8:5822

Change-Id: I9302a07971cfd32350d69b1b8f182d0ba7245b77
Reviewed-on: https://chromium-review.googlesource.com/454018Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43857}