aix: add FAT library support for libstdc++fs.a

aix: add FAT library support for libstdc++fs.a

Make libstdc++fs.a a 32/64 bit FAT library.

It is too complicated for the FAT library Makefile fragment to determine if
--enable-libstdcxx-filesystem-ts has been enabled.  This patch checks the
existence of libstdc++fs.a at build time and adds the complementary object
file if it was built.

libstdc++-v3/Changelog:

2020-10-11  Clement Chigot  <clement.chigot@atos.net>

	* config/os/aix/t-aix: Add complementary mode object file to
	libstdc++fs.a