aix: Use $(AR) without -X32_64 to build FAT libraries.

AIX FAT libraries should be built with the version of AR chosen by configure.
The GNU Make $(AR) variable includes the AIX -X32_64 option needed
by the default Makefile rules to accept both 32 bit and 64 bit object files.
The -X32_64 option conflicts with ar archiving objects of the same name
used to build FAT libraries.

This patch changes the Makefile fragments for AIX FAT libraries to use $(AR),
but strips the -X32_64 option from the Make variable.

libgcc/ChangeLog:

2020-09-27  Clement Chigot  <clement.chigot@atos.net>

	* config/rs6000/t-slibgcc-aix: Use $(AR) without -X32_64.

libatomic/ChangeLog:

2020-09-27  Clement Chigot  <clement.chigot@atos.net>

	* config/t-aix: Use $(AR) without -X32_64.

libgomp/ChangeLog:

2020-09-27  Clement Chigot  <clement.chigot@atos.net>

	* config/t-aix: Use $(AR) without -X32_64.

libstdc++-v3/ChangeLog:

2020-09-27  Clement Chigot  <clement.chigot@atos.net>

	* config/os/aix/t-aix: Use $(AR) without -X32_64.

libgfortran/ChangeLog:

2020-09-27  Clement Chigot  <clement.chigot@atos.net>

	* config/t-aix: Use $(AR) without -X32_64.