aix: Fix building fat library for AIX

aix: Fix building fat library for AIX

With the change in subdirectories, the code for libgfortran fat libraries
needs to be adjusted to explicitly reference the subdirectory.  AIX
creates fat library archives and the compiler itself can be built as
either 32 bit or 64 bit application and default code generation.  For
the two, alternate versions of the compiler to interoperate, GCC needs
to construct the fat libraries manually.

The Makefile fragment had been trying to leverage as much of the existing
targets and macros as possible.  With the subdirectory change, the
location of single.o is more obscured and cannot be determined without
libtool.  This patch references the location of the real object file
more explicitly.

Utilizing subst seems like overkill and unnecessary obscuration for a single
object file.  Either way, it's digging below the libtool abstraction layer.

This also fixes Fortran bootstrap on AIX.

Bootstrapped on powerpc-ibm-aix7.3.0.0

libgfortran/ChangeLog:

	* config/t-aix (all-local, libcaf_single): Explicitly reference
	caf/.libs/single.o

Signed-off-by: David Edelsohn <dje.gcc@gmail.com>