util: fix formatting of objects with SIMD enabled

When SIMD is enabled, `util.format` couldn’t display objects
(with at least 1 key) because the formatter function got
overridden.

PR-URL: https://github.com/nodejs/node/pull/7864


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