aix: Remove AIX 6.1 support.

AIX 6.1 is past end of life and extended support.  This patch removes
the configuration option and references to AIX 6.1.

contrib/ChangeLog:

	* config-list.mk: Remove rs6000-ibm-aix6.1.
	Rename rs6000-ibm-aix7.1 to powerpc-ibm-aix7.1.
	Add powerpc-ibm-aix7.2.

gcc/ChangeLog:

	* config.gcc (powerpc-ibm-aix6.*): Remove.
	* config/rs6000/aix61.h: Delete.