aix: Alias -m64 to -maix64 and -m32 to -maix32.

aix: Alias -m64 to -maix64 and -m32 to -maix32.

GCC on AIX historically has used -maix64 and -maix32 to switch to 64 bit mode
or 32 bit mode, unlike other ports that use -m64 and -m32.  The Alias()
directive for options cannot be used because aix64 is expected in multiple
parts of the compiler infrastructure and one cannot switch to -m64 due to
backward compatibility.

This patch defines DRIVER_SELF_SPECS to translate -m64 to -maix64 and
-m32 to -maix32 so that the command line option compatible with other
targets can be used while continuing to allow the historical options.

gcc/ChangeLog:

	* config/rs6000/aix.h (SUBTARGET_DRIVER_SELF_SPECS): New.
	* config/rs6000/aix64.opt (m64): New.
	(m32): New.