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: #9645

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

configure

@@ -677,7 +677,7 @@ def host_arch_cc():
  '__MIPSEL__' : 'mipsel',
  '__mips__' : 'mips',
  '__PPC64__' : 'ppc64',
- '__PPC__' : 'ppc',
+ '__PPC__' : 'ppc64',
  '__x86_64__' : 'x64',
  '__s390__' : 's390',
  '__s390x__' : 's390x',