* config.gcc (powerpc-ibm-aix7.1.*): New stanza.
(powerpc-ibm-aix[789]*): Default to AIX 7.2.
* config/rs6000/aix71.h (TARGET_DEFAULT): Revert to Power4 ISA.
* config/rs6000/aix72.h: New file.

From-SVN: r258082