test: make arch available in status files

The value is retrieved from `process.arch` in node itself.

PR-URL: https://github.com/nodejs/node/pull/5997


Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>