Rename ICU NaCl targets

Use _nacl suffix for NaCl targets instead of _untrusted.

R=mark@chromium.org

Review URL: https://codereview.chromium.org/129893002

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/icu46@243949 4ff67af0-8c30-449e-8e8b-ad334ec8d88c