src: fix build for older clang

Removes use of builtins that are unavailable for older clang. Per
benchmarks, only uses builtins on Windows, where speedup is
significant.

Also adds test for unaligned ucs2 buffer write. Between #3410
and #7645, bytes were swapped twice on bigendian platforms if buffer
was not two-byte aligned. See comment in #7645.

PR-URL: https://github.com/nodejs/node/pull/7645
Fixes: https://github.com/nodejs/node/issues/7618


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>