build: default to ppc64 on AIX

The ./configure python script searches `gcc -dM -E -` for the ARCH
flags. On AIX, gcc builds in 32 bit mode by default prior to gcc v6, so
you don't get the __PPC64__ flag unless you run `gcc -maix64 -dM -E -`.

We don't support ppc 32 bit for any OS, so always use ppc64 as the
host_arch.

PR-URL: https://github.com/nodejs/node/pull/9645



Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>