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',