src: remove __builtin_bswap16 call

Not supported by apple-gcc and I'm not convinced it's worth adding more
preprocessor hacks when it should be easy as pie for the compiler to
to optimize the byteswap.  If it doesn't, fix the compiler.

Fixes: https://github.com/nodejs/node/issues/4284
PR-URL: https://github.com/nodejs/node/pull/4290


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>