[Patch Ada/Darwin] factor LIBGNAT_TARGET_PAIRS for darwin sub-targets.

[Arnaud Charlet]

> No functional change, just factor out the common LIBGNAT_TARGET_PAIRS
> across the port.
> OK for trunk?

OK

> ada:
> 
> 	* gcc-interface/Makefile.in (Darwin): Factor LIBGNAT_TARGET_PAIRS
> 	across the port.