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 v0.12 should do
this too for compatibility.

PR-URL: https://github.com/joyent/node/pull/9375


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