buffer: reapply 07c06672

Original commit message:

    buffer: align chunks on 8-byte boundary

    When slicing global pool - ensure that the underlying buffer's data
    ptr is 8-byte alignment to do not ruin expectations of 3rd party C++
    addons.

    NOTE: 0.10 node.js always returned aligned pointers and io.js should
    do this too for compatibility.

PR-URL: https://github.com/nodejs/node/pull/2487


Reviewed-By: Trevor Norris <trev.norris@gmail.com>