util: support classes in util.deprecate()

Classes cannot be instantiated without new, but util.deprecate()
uses Function.prototype.apply(). This commit uses new.target to
detect constructor calls, allowing classes to be deprecated.

PR-URL: https://github.com/nodejs/node/pull/7690


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>