Swaping the order when loading surrogate pairs on big endian
platform. This fixes testcase string-iterator.js on big
endian.

BUG=
R=bmeurer@chromium.org, mstarzinger@chromium.org, jkummerow@chromium.org

Review-Url: https://codereview.chromium.org/2431223010
Cr-Commit-Position: refs/heads/master@{#40535}