aix: Add FAT library support for libffi for AIX

[CHIGOT, CLEMENT]

> Even if GCC64 is able to boostrap without libffi being a
> FAT library on AIX, the tests for "-maix32" are not working
> without it.
> 
> libffi/ChangeLog:
> 2021-10-21  Clément Chigot  <clement.chigo@atos.net>
> 
>       * Makefile.am (tmake_file): Build and install AIX-style FAT
>          libraries.
>       * Makefile.in: Regenerate.
>       * include/Makefile.in: Regenerate.
>       * man/Makefile.in: Regenerate.
>       * testsuite/Makefile.in: Regenerate.
>        * configure (tmake_file): Substitute.
>        * configure.ac: Regenerate.
>        * configure.host (powerpc-*-aix*): Define tmake_file.
>        * src/powerpc/t-aix: New file.
>
> I've already made a PR to libffi itself in order to add the common part of 
> this patch to it. But for now, it's still unmerged: 
> https://github.com/libffi/libffi/pull/661. 

Gentle ping, 

Thanks
Clément