aix: FAT libraries: test native compiler mode directly

The FAT libraries config fragments need to know which library is native
and which is a multilib to choose the correct multilib from which to
append the additional object file or shared object file.  Testing the
top-level archive is fragile because it will fail if rebuilding.  This
patch tests the compiler preprocessing macros for the 64 bit AIX specific
__64BIT__ to determine the native mode of the compiler in MULTILIBTOP.

2020-07-14  David Edelsohn  <dje.gcc@gmail.com>

libatomic/ChangeLog

	* config/t-aix: Set BITS from compiler cpp macro.

libgcc/ChangeLog

	* config/rs6000/t-slibgcc-aix: Set BITS from compiler cpp macro.

libgfortran/ChangeLog

	* config/t-aix: Set BITS from compiler cpp macro.

libgomp/ChangeLog

	* config/t-aix: Set BITS from compiler cpp macro.

libstdc++-v3/ChangeLog

	* config/os/aix/t-aix: Set BITS from compiler cpp macro.