buffer: fix performance regression

V8 5.4 changed the way that the default constructor of derived classes
is called. It introduced a significant performance regression in the
buffer module for the creation of pooled buffers. This commit forces the
definition back to how it was implicitly before.

Ref: https://bugs.chromium.org/p/v8/issues/detail?id=4890

PR-URL: https://github.com/nodejs/node/pull/8754


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>