buffer: handle UCS2 `.fill()` properly on BE

There was a byte-order mismatch for `buffer#fill` on big-endian
platforms. Weirdly, the tests seemed to expect that wrong behaviour.

PR-URL: https://github.com/nodejs/node/pull/9837


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>