build: fix DESTCPU detection for binary target

`make binary` attempts to auto detect DESTCPU if not set, but was
assuming being on an Intel architecture.

PR-URL: https://github.com/nodejs/node/pull/6310


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>