Fix SkBlitRow_opts_arm so that it works on ARM v4t.
Original Mozilla bug: https://bugzilla.mozilla.org/show_bug.cgi?id=901208

R=reed@google.com, mtklein@google.com, reed1
BUG=skia:

Author: george@mozilla.com

Review URL: https://codereview.chromium.org/337853003

Change-Id: I822ef24123f2ae10815d99f25d07ee06e2c47af9