Add Ada support for aarch64-*-freebsd*

[Arnaud Charlet]

> The attached patch to gcc trunk enables Ada support on
> aarch64-*-freebsd*.  All tests pass except those affected by the
> currently missing unwind support (c52103x, c52104x, c52104y,
> cb1010a, cb1010c, cb1010d,  gnat.dg/null_pointer_deref* (3)).
> 
> I'd also like to get this patch backported to the gcc-6 branch.
> There's a one-line content difference between patches, so I've
> attached the 6.x specific patch as well.
> 
> My copyright assignment is on file and in order.
> 
> gcc/ada/Changelog:
> 2017-02-XX  John Marino  <gnugcc@marino.st>
> 
>         * gcc-interface/Makefile.in: Support aarch64-freebsd

It seems wrong to reuse system-freebsd-x86.ads for a different processor.
We should either:

- use a different system file (e.g. system-freebsd-aarch64.ads)
- or rename the file to e.g. system-freebsd.ads, *and* adjust the
  setting of at least Default_Bit_Order to something like:

     Default_Bit_Order : constant Bit_Order :=
                         Bit_Order'Val (Standard'Default_Bit_Order);

Arno