aix: SYSTEM_IMPLICIT_EXTERN_C

aix: SYSTEM_IMPLICIT_EXTERN_C

AIX 7.3 system headers are C++ safe and GCC no longer needs to define
SYSTEM_IMPLICIT_EXTERN_C for AIX 7.3.  This patch moves the definition
from aix.h to the individual OS-level configuration files and does not
define the macro for AIX 7.3.

The patch also corrects the definition of TARGET_AIX_VERSION to 73.

gcc/ChangeLog:
	* config/rs6000/aix.h (SYSTEM_IMPLICIT_EXTERN_C): Delete.
	* config/rs6000/aix71.h (SYSTEM_IMPLICIT_EXTERN_C): Define.
	* config/rs6000/aix72.h (SYSTEM_IMPLICIT_EXTERN_C): Define.
	* config/rs6000/aix73.h (TARGET_AIX_VERSION): Increase to 73.