console: allow Object.prototype fields as labels

Console.prototype.timeEnd() returns NaN if the timer label
corresponds to a property on Object.prototype. This commit
uses Object.create(null) to construct the _times object.

Fixes: https://github.com/joyent/node/issues/9069
PR-URL: https://github.com/joyent/node/pull/9116


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