lib,src: make os.endianness() inlinable

Turn os.endianness() from a run-time function into a pure JS function.
Upsides: makes it a good candidate for inlining at the call site.
Downsides: none that I can think of.

PR-URL: https://github.com/node-forward/node/pull/55


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>