buffer: fix buffer alignment restriction

Recent phantom weakness API changes to buffer, ebbbc5a7, ending up
introducing an alignment restriction on the native buffer pointers.
It turns out that there are uses in the modules ecosystem that rely
on the ability to create buffers with unaligned pointers (e.g.
node-ffi).

It turns out there is a simpler solution possible here. As a side
effect this also removes the need to have to reserve the first
internal field on buffers.

PR-URL: https://github.com/nodejs/node/pull/5752


Reviewed-By: trevnorris - Trevor Norris <trev.norris@gmail.com>
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>